Comparative Dent a 00 Thom

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DIVISION OF PHYSICAL ANTHROPOLOGYU. S. NATIONAL MUSEUM

THE HRDLICKA LIBRARYV V

Dr. Ales Hrdlicka was placed in charge of theDivision of Physical Anthropology when it was firstestablished in 1903. He retired in 1942. During thistime he assembled one of the largest collections ofhuman skeletons in existence and made outstandingcontributions to his science. On his death, September5, 1943, he bequeathed his library to the Division, wt+k-the provision that " jf be kept exclusivelyin the said Division, where it may be consultedbut not loaned out "


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COMPARATIVE DENTAL ANATOMY


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COMPARATIVEDENTAL ANATOMY

BYALTON HOWARD THOMPSON, D.D.S

Late Professor of Dental Anatomy, Human and Com-parative, in the Kansas City Dental College,Kansas City, Mo.

SECOND EDITIONREVISED BY

MARTIN DEWEY, D.D.S., M.D.,Formerly Head of the Department and Associate Professor

of Orthodontia, University of Iowa; FormerlyProfessor of Dental Anatomy and Orthodontia,Kansas City Dental College, etc.

ILLUSTRATED

(A**S*(Si.V

ST. LOUISC. V. MOSBY COMPANY

1922


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Copyright. 1915, by C. V. Mosby Company

Press ofC. V. Mosby Company

St. Louis


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TOHELEN MOON THOMPSON

Whose Devotion and Attention to Dr. AltonHowaed Thompson in His Last Days

Were an Inspiration to HisMany FriendsTHIS VOLUME IS DEDICATED.


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PREFACEsidelights it throws upon human odontography, as toboth tooth forms and functions, as well as for the scien-tific study of the evolution and philosophy of toothforms. The time was when it was necessary to apolo-gize for the intrusion of Comparative Dental Anatomyinto the curriculum of dental education; but it is amatter of congratulation that the value of this branchas an element in our professional education is now gen-erally recognized. The study of the forms and func-tions of the teeth of other animals than man, as a meansof conveying a better understanding of the forms andfunctional purposes of the human teeth, is now fullyappreciated. It is also recognized that this study fur-nishes the only scientific elucidation of the origin andprinciples of these forms and functions, which had here-tofore been taught by the study of the human teeth alone.In regard to the general scheme of the book, it mustbe stated that some liberties have been taken with theusual zoological classifications in order to have an ar-rangement in a scheme that would be harmonious withthe progressive advance of the perfecting of tooth forms,for convenience of description. It is to be hoped thatthis breach will be overlooked, as well as the zoologicalerrors that may have crept into the pages, but whichwill probably not affect the value of the lessons to bedrawn from the main principles.While this book will furnish the various facts and

principles of Comparative Dental Anatomy, it will benecessary for the teacher to enlarge upon and elaboratethe subject by the use of the general works of the an-atomists and zoologists. It will also be necessary to illus-trate the lessons by the use of accessories such as skulls,charts, sketches, and especially the lantern, which is thebest of all for illustration in the class-room.


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PREFACEThe best place for this branch in the curriculum will

be as a preliminary study in the course on Dental Anat-omy, preceding and leading up to human dental anat-omy. It begins with the lowest form of life and leads upto the highest in regular gradation,taking the teethseriatim from the lowest types, and showing their pro-gressive evolution from simple to complex forms.

It would be impracticable to append references to au-thorities in a condensed work of this character, foreconomy of space; but the writer takes pleasure in ac-knowledging his indebtedness to the leading authoritiesupon odontography and zoology for the many drafts hehas made upon the rich stores they have accumulatedand placed at the disposal of students and teachers.He wishes also to acknowledge the courtesy of Mr. C. H.Ward, of Ward's Natural History Establishment, Roch-ester, N. Y., who kindly furnished specimens for mostof the illustrations.

A. H. Thompson.Topeka, Kansas.


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PKEFACE TO SECOND EDITIONIn a conversation with Dr. Thompson several months

before his death, we discussed the advisability of the re-publication of his book on Comparative Dental Anatomywhich had been out of print for some time.At Dr. Thompson's request I undertook the task of

preparing this new edition, feeling that there was a needamong dental students and teachers for a text upon thissubject.Such changes as have been found necessary in this

edition have the full approval of Dr. Thompson. Con-flicting theories have been omitted and we have at-tempted to state only proven facts. Many new andoriginal illustrations have been added, but we have triedto use only those which show some interesting feature inthe evolution of the dental apparatus.A number of roentgenograms are shown which weremade by Dr. E. H. Skinner from specimens in my pri-vate collection.

If this volume should prove an aid to the student andteacher of dentistry, it will have served its purpose.

Martix Dewey.


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CONTENTSPAGECHAPTER I

General Zoology and Comparative Anatomy . . . . 17CHAPTER II

The Teeth in General 26CHAPTER III

The Teeth of Invertebrates 33CHAPTER IV

The Teeth of Vertebrates 42CHAPTER V

The Teeth of Fishes 85CHAPTER VI

The Teeth of Reptiles 96CHAPTER VII

The Teeth of Mammals 115CHAPTER VIII

The Teeth of Mammals (Continued) 154CHAPTER IX

The Teeth of the Higher Apes and Man 182Glossary . ' 205Index 217


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COMPARATIVE DENTALANATOMYCHAPTEE IGENERAL ZOOLOGY AND COMPARATIVEANATOMY

The Animal Kingdom is divided into two sub-kingdoms,viz., (a) Invertebrates and (b) Verte-brates. These sub-kingdoms are further sub-divided into classes; classes are divided into or-ders; orders into families; families into genera;and genera into species. Species is the last divi-sion into which animals can be classified, but ifthe individuals of a species vary much from thenormal type, they may be classed as sub-speciesor varieties. All animals are grouped with refer-ence to their plan of structure, and classificationis made according to the system of organization,and without regard to superficial characters orresemblances except so far as external featuresmay have reference to functions.

Vertebrates and Invertebrates are distin-guished from each other by the presence or ab-sence of a vertebral column or backbone. The

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18 COMPARATIVE DENTAL. ANATOMYVertebrates have a cerebro-spinal axis and astrong bony column composed of separate piecescalled vertebrae, which are connected together byligaments and are more or less movable. Thesub-kingdom of the Invertebrates comprises allclasses of animals which do not have a vertebralcolumn, whatever their various plans of struc-ture may be. The class of the Vertebrates istherefore homogeneous, and that of the Inverte-brates very heterogeneous.The sub-kingdom of the Invertebrates includes

all animals which have no internal backbone orvertebral column; such as the Infusoria, Hy-droids, Eadiata, Worms, Insecta, Crustacea, Mol-lusca, etc.There is no spinal cord with its anterior en-

largement, the brain in Invertebrates, but insteadthe nervous system consists of chains of gangliascattered throughout the system, arranged inrows or circles connected by cords of nervoussubstance and giving filaments to various partsof the organism.The digestive system is simple. The stomach

may be a single sac with but one opening as inthe Hydroids, or a complete alimentary canal withtwo openings,the oral and anal,as in theworms, insects, mollusks, etc. In the lower formsspecialized digestive glands do not appear, butare present in the higher orders.


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GENERAL ZOOLOGY AND COMPARATIVE ANATOMY 19The circulation is a mere water vascular sys-

tem in the lowest aquatic forms, in which thereis no corpusculated or true blood, and no circula-tory organs. In the higher Invertebrates thereis true blood, colorless or greenish, with true veinsand arteries. In the insects there is a distinctheart with one ventricle. In the mollusca there isa heart with one valve which propels the bloodboth ways alternately. Some forms have a biloc-ular heart.

Eespiration is performed in the lowest formsby tentacles or cilia, and the higher aquatic formshave cilia or gills. In the insects the blood isaerated by circulation of the air in the pulmonarytubes which ramify throughout the body. Thesnails breathe by means of an air-sac with aciliated lining.Locomotion is performed by various means : by

tentacles and cilia in the lower forms; by legsand wings in the insects ; by legs in the Crustaceaby a fleshy peduncle in the mollusca, etc.

Eeproduction is performed by fission, budding,etc., in the lowest forms; the laying of gelatinouseggs in the higher orders, etc. Some forms, asthe insects, undergo a series of metamorphosesbefore attaining the mature stage.The sub-kingdom of the Vertebrates comprises

all animals which have an internal backbone orvertebral column composed of articulated verte-


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20 COMPARATIVE DENTAL ANATOMYbrae. It includes the Fishes, Reptiles, Birds, andMammals. From the vertebral column the limbsare suspended, and by it the vital organs areheld in place. It is the central structure andmainstay of the framework of the body.A transverse section of a vertebrate body re-veals two cavities or tubes, which are separatedby the vertebral column. The upper cavity orcanal, which is formed by the arches of the verte-brae, contains the spinal cord and brain, and sois called the neural arch or cavity. The lowerand larger cavity or tube is below the vertebralcolumn, is formed by the ribs and abdominal

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Fig. 1.Section of Vertebrate, a, The neural arch; b. The visceral arch.walls, and contains the vital organs: the viscera.Hence it is called the visceral arch or cavity.The nervous system of Vertebrates consists of

the spinal cord and the brain, which is coveredby the especially developed cranium. Nervebranches and filaments are sent from the cerebro-spinal axis to all portions of the body. Many


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GENERAL ZOOLOGY AND COMPARATIVE ANATOMY 21of the lower forms of Vertebrates, as the Am-phioxus, have no bony spinal column, but onlya cartilaginous structure; but the spinal cord ispresent, looking like the notochord of the embryosof all Vertebrates in the first stages of existence.The alimentary canal has its beginning at the

oral opening, the mouth, which is armed by theteeth of a great variety of forms in the Fishes,Eeptiles, and Mammals for the securing and re-duction of food preparatory to digestion. Thereis a digestive stomach and an intestinal canal,which is more or less complicated in the differentclasses, that leads to the anal opening at theposterior extremity of the organism.The circulation is complete in all forms of

Vertebrates. The blood is corpusculated, and isgenerally red in color. The heart has from twoto four chambers.

Respiration is performed by gills in the Fishes,by gills and lungs in Eeptiles, and by lungs onlyin the Birds and Mammals.The external covering in this sub-kingdom pre-

sents a great variety of forms. In the Fishes andReptiles the skin is bare or is protected by scalesor spines of great variety in size and shape. InMammals there is a tough leathery skin or dermalplates, or fur, hair, or bristles. The birds arecovered with feathers, which on the wing andtail are enlarged and modified to assist in the


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22 COMPARATIVE DENTAL ANATOMYperformance of aerial flight. The limbs in theVertebrates are suspended from the vertebralcolumn, and constitute the appendicular skeleton.There are never more than four limbs, sometimesthey are reduced to two, and in the snakes areabsent entirely. When present they are modifiedto perform a variety of functions : such as swim-ming in the water, flying in the air, running uponthe ground, climbing trees, seizing objects, etc.Eeproduction is performed by laying gelatinous

eggs by the fishes and batrachian reptiles byeggs with a more or less hard shell in the higherreptiles and birds,hence oviparous,and by theyoung being born alive in some reptiles andnearly all mammals,hence viviparous.Comparative Anatomy is the study and com-

parison of the anatomy of lower animals withthe anatomy of Man.The comparative method of study is the onlyscientific method, for one branch cannot be studied

alone, but must be illuminated by comparison withkindred branches. Man is but an insignificantpart of nature, and is connected in the closest waywith the animal kingdom. His body is identicalwith those of animals in its functions, and withall Vertebrates, especially mammals, in its struc-ture. Therefore his anatomy can only becometruly scientific through comparative anatomy andhis physiology through comparative physiology.


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GENERAL ZOOLOGY AND COMPARATIVE ANATOMY 23The structure and functions of his organs areonly to be fully understood by comparison withthose of lower animals.The leading principles of comparative study

are Homology and Analogy. In biology thoseorgans or parts in different animals are said tobe analogous which, however different theirorigin, have a general similarity of form andespecially of function, while those are calledhomologous which, however different their gen-eral appearance and however various their func-tions, are but modifications of the same part al-tered for different purposes. For example, thewing of the bird and the wing of the butterflyare analogous organs, for they look somewhatalike and have the same function,flying; butthey are not homologous, for they are not thesame in structure and are dissimilar in origin.But the forelimbs of all Vertebrates, whether thefcrepaws of a reptile or a mammal, the wings ofa bird or bat, the arm of a man, the flipper of awhale,though so different in form and function,are homologous parts. They have the samegeneral structure, are composed of the samepieces and undoubtedly have the same origin;they are but modifications of the same structurefor different functions. They are homologousbut not analogous parts. Again, the lungs of amammal and the gills of a fish are analogous or-


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24 COMPARATIVE DENTAL. ANATOMYgans, since they have the same function,theaeration of the blood,bnt they are not the sameorgans. Therefore, homology has reference tocommunity of origin, and analogy to similarity offunction only.Comparative Dental Anatomy is the study of

the teeth of lower animals as compared to man.The observation and comparison of their formsand functions will illustrate the understanding ofthe human teeth, because the teeth of man canonly be studied scientifically by comparison withlower forms of the same organs,just as in study-ing other organs, by the comparative method.Thus homologies with the teeth of lower formscan be demonstrated, and analogous structuresin other locations will throw light upon their vari-ations, by these studies. This will illustrate theprinciples which have controlled their growthand organization, and explain the varied detailsof function.The teeth of man have, in the course of their

evolution to present forms, passed through thetransitional stages common to all organs in allanimals. In consequence they, like other organs,still retain many features that indicate relation-ship with the teeth of lower animals. The teethof man have been much reduced in size andstrength, and are more or less rudimentary andmuch less specialized, as compared with the highly


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GENEEAL ZOOLOGY AND COMPAEATIVE ANATOMY 25developed teeth of other animals. Comparativestudy will therefore point out the relationship ofthe teeth of man, indicate the path of their de-velopment, and explain the causes of their presentdegradation.


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CHAPTEE IITHE TEETH IN GENEEAL

Definition. The term teeth is applied to allhard, usually calcined substances placed at theorifice of the alimentary canal. They are gen-erally confined to the cavity of the mouth, but aresometimes found in the pharynx and rarely inthe oesophagus. The name is, however, confinedto those structures located in the oral cavity whichcontain a calcified tissue known as dentin.

Origin. The teeth of animals are derived fromthe layers of the skin and hence are but special-ized dermal structures. In the Invertebrates,they are evolved from the superficial layers of thederm, and are therefore called ecderonic, whilethe teeth of Vertebrates are derived from thedeeper portions, the corium of the integument,and are hence called enderonic. The dentin isderived from the mesoderm, but the enamel is acalcified substance derived from epithelium, whichis readily demonstrated by its histological ele-ments.

Tissues. The dental tissues are three in num-ber, enamel, dentin and cementum. The cemen-

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THE TEETH IN GENERAL 27turn is a mere osseous tissue, and in all of its ele-ments resembles true bone. It surrounds theroots of the teeth and is in contact with the alveolo-dental periosteum or peridental membrane. Thedentin constitutes the main body of the tooth, andis but modified bone as to its histological elements.It has the same organic basis as bone,i.e., gluten.In many lower forms it is the only tissue of thetooth, the enamel not being yet organized. In thehigher forms the enamel is the main working ele-ment of the tooth, for which the other tissues aremere supports. It appears to have been devel-oped to supply the demand for a more resistingstructure as the function of mastication becamemore specialized. It is developed from the epi-thelium, and consists of calcified rods, with theorganic basis : keratin, like horn, nails, hairs, andother epithelial structures.Functions of the Teeth. The functions of theteeth can be divided into primary and secondary.The primary function of the teeth is the secur-

ing and preparing of the food for digestion andassimilation. For this purpose they were calledinto existence and such modifications as we findin animals is always one which will enable theanimal to eat a certain class of food. It has beensaid that the food of the animal has been respon-sible for the change in the shape of the teeth;whether this is exactly true or not, we can say


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28 COMPARATIVE DENTAL ANATOMYwithout fear of contradiction, that only those ani-mals have existed whose dental apparatus hasbeen able to change to meet their needs. We findmany fossils, which have disappeared becausetheir dental apparatus was not suited to their par*ticular needs.The first function in the securing of the food

is prehension, or the seizing of the food sub-stances. It is the only function which the teethperform in some fishes and reptiles. The secondto appear in the animal kingdom was deglutition.Deglutition is the principal function of the teethlocated on the vomer and palates of most of thefishes and is one of the principal functions of theteeth of the non-poisonous snakes. Incision, orthe cutting of the food into pieces, was the nextfunction to appear. In some animals, we findincision has to a certain extent taken the placeof prehension; this may be said of the teeth ofman, as the incisors are well developed. In someof the lower animals, the anterior teeth are shapedlike incisors, an example of which is the sargus.The next step or function was the crushing ofthe food, which was first done by blunt flat teethor by pavement teeth. Then came the functionof grinding the food or mastication, which wasperformed by teeth of various shapes, dependingupon the class of food. Insalivation is the lastfunction of the teeth and is the mixing of the


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THE TEETH IN" GENERAL 29food with the saliva. This function is also per-formed by the tongue and cheeks.Of the secondary functions of the teeth, war-fare is the most important. The canines of thecarnivora are developed for prehensile purposesbut they are also used in combat and to destroyanimal life. The nature of an animal can be toldby the teeth. Associated with warfare for pro-tection and securing of the food is sexual war-fare. We find the teeth of some animals are bet-ter developed in the males than in the females.In some animals we have the function of sexualattraction closely associated with sexual warfare.The canines are well developed in some males,e.g., the boar, but the growth of those teeth ischecked by castration and the teeth of the cas-trated males become no larger than the teeth ofthe females. Teeth are also used as tools, anexample of which is the beaver, which uses theteeth to gnaw off trees which are used in themaking of their dams and houses. The loweranterior teeth of the lemur are used to dress theirfur and have a modified cosmetic function. Inman we find the teeth have a function in speech;they also have an esthetic function and throughthe esthetic function also have a function of sex-ual attraction.The food-reducing mechanism of animals pre-

sents great variety when viewed throughout the


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30 COMPARATIVE DENTAL ANATOMYentire animal kingdom. In no set of organs isthe invention of nature so varied or the capacityfor change so great as in the teeth. This varia-tion is due to the modifying influences of thequalities of the various substances employed byanimals for food, for the teeth and jaws areadapted to the particular manner of reductionthat the food of each species requires. It is anadaptation of tools to material, not of materialto tools. Therefore there has arisen, in responseto the demands of food selection, a great varietyof forms of teeth. 1 The different kinds of foodhave dictated the different kinds of tooth forms.The force that dictates the tooth form is still

a matter of some dispute. The older writers be-lieved that the shape of the tooth was influencedby the movement of the jaw and a study of fos-sils seems to bear that out. However, a studyof the embryology and histology of the teeth andjaw seems to indicate that the shape of the toothhas been responsible for the shape of the jaw andthe shape of the temporo-mandibular articulation.The development of the tooth precedes the de-velopment and final shaping of the jaws. Thusin the carnivorous animals we find a tooth whichis long and the cusps of which are sharp, themovement of the mandible is vertical and the

l Teeth which are suited to the particular food have enabled the animalto live, but there may have been many animals of the past whose teeth didnot change and the animals perished. Editor.


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THE TEETH IN GENERAL 31temporo-mandibular articulation is a mere hinge,admitting of opening and closing of the jaw with-out any lateral motion. The jaw is short andstout to sustain the force of hard biting, and theteeth are developed vertically, with long pointsand blades, in such a manner as to resist thegreatest strain. In the other extreme formthe Herbivorous mammaliathe teeth are broadand flat and we find the temporo-mandibular artic-ulation is open to allow extreme lateral movementof the mandible. The jaw bones are light and thesurfaces of the teeth are roughened to producea good grinding surface and there is less strainon the jaws than in the carnivorous animals. Inthe elephant we find the dental tissue arrangedin plates and the tooth is more effective by anantero-posterior movement and the jaws and ar-ticulation are arranged for that purpose. So inall animals there is a close relation existing be-tween the shapes of the teeth and the jaw move-ments.Tooth Forms. The original and primitive

form of the tooth is that of the single, simple cone,as illustrated in the teeth of fishes and reptiles,which are simple cones with but little modifica-tion. From this primitive form all other formshave been derived by modification and duplica-tions of the single cone. Thus the incisors ofman are formed of a single cone, the base of


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32 COMPAKATIVE DENTAL ANATOMYwhich is compressed to form the wide cuttingedge. The canine is a single cone, the base ofwhich is compressed into a trihedral, pointedprism. The bicuspids are formed of two conesfused together,the base rounded to make thecusps,and two cones are distinct the entirelength of the tooth. The typical upper molar isformed by the coalescence of three cones, whichare plainly marked, and the lower molar of fourcones. Thus the teeth of all animals, even thehighly complex and specialized tooth of the highermammals, are evolved. As Cope says, "Thetransition from single to complex teeth is ac-complished by repetition of the simple cone in va-rious directions. First, there are cylindrical in-cisors, then flat ones, then divided roots; theninternal repetition of a root and cusp; then pos-terior repetition. Very complex teeth, as multi-tubercular molars, are formed by both posteriorand lateral repetition.'' Thus the primitive formof tooth is that of a simple cone, from which allsubsequent forms, however complex, have beenderived by repetition, duplication, and modifica-tion of cones and cusps.


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CHAPTER IIITHE TEETH OF INVERTEBRATES

The teeth of this sub-kingdom present as manyvariations and extraordinary forms as the or-ganic designs of the heterogeneous mass of ani-mals composing this great division. Not manyof these lower forms possess teeth, but when pres-ent the form presented and the analogies sug-gested are very instructive. They are analogousto the teeth of Vertebrates, as the teeth are usuallyoral organs in the Invertebrates and perform thesame functions as in the higher sub-kingdom,i.e., the prehension and reduction of food, pre-paratory to digestion; but they are not homolo-gous with the teeth of Vertebrates, however, asthey do not have the same origin or structure.Almost every group, in some of its forms, ex-hibits some sort of a dental apparatus for thereduction of food, though few are homologouswith true teeth. Most of the lower forms arewithout a masticating armature.

Origin. The food-reducing mechanism of In-vertebrates, whether oral organs or modifiedlimbs, is composed of calcified connective tissue

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34 COMPARATIVE DENTAL ANATOMYor of chitin, and is derived from the superficiallayer of the derm. They are therefore ecderonic.In insects and crustaceans the food apparatus ismodified from the chitinoid external covering.The forms of the food apparatus present great

and heterogeneous variety in this division. Theso-called teeth of Invertebrates are often but ser-rated jaws placed about the oral opening. Themargin of the mouth may be raised into folds andarmed with cuticular plates. In the insects andCrustacea the jaws and modified limbs are formedfrom the exo-skeleton. In someas the cuttle-fishthere is a strong beak. In the Sea-urchinthere are five teeth set in true alveoli. In themollusks the teeth are supported by a movableband, called the odontophore.

Functions. Prehension is performed in thissub-kingdom by cilia in many of the lower forms,in the worms and gastropods by a suctorial formof the mouth, in others by tentacles, and in insectsby their chitinoid jaws and modified limbs. Cut-ting and dividing food is performed by jaws andmastication by gizzards, when performed at all.No true masticating teeth exist in the entire sub-kingdom. Some have the food apparatus devel-oped for the purposes of combat or sexual at-traction, some for drilling through shells to getthe juices of the animal within, or even to drillinto rock.


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36 COMPARATIVE DENTAL ANATOMYother, so as to insure continued sharpness fromwear. They are set in alveoli of bony structure,and are moved by sets of strong muscles in vari-ous directions. The entire apparatus consists oftwenty pieces,i.e., five teeth, five alveoli, fiverotalse, and five radii. It is concealed within thetest in life with only the points of the teeth pro-jecting, which are very effective for cutting shells,boring into rocks, and reducing food substances.The Annuloida comprise the segmented worms,

some of which possess so-called teeth, but thesepartake more of the nature of serrated jaws thanof teeth. These jaws are located on the secondor buccal segment, which may be protruded fromthe mouth a considerable distance. These jawsare of chitinous structure, commonly paired andof an infinite variety of forms. In the Leeches,the mouth is provided with three lenticular jaws,with the projecting edges finely serrated. Themedicinal leech has two rows of serrations, whichmake three radiating slits. The strong suctorialpower draws an eminence of skin into the mouth,which is slit by the serrated jaws.The Nereis and Philodace have strong jaws like

the carnivorous beetles, which are cruelly effectivein attacking lower Invertebrates (Fig. 3). Theirjaws are serrated and opposite, and are workedby powerful muscles.

In the Arthropodaincluding the Insects and


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THE TEETH OF INVEETEBEATES 37Crustaceawe have an approacli to true jaws,but they work laterally instead of vertically, asin the Vertebrates. The mandible and maxillaeare very dense chitinous material, and the"teeth" are merely serrations on the edges. Inthe insects one pair of eachmandibles andmaxillaemake four jaws, which work trans-

Fig. 3.Head and Jaws of Nereis virens.versely in addition to the labia, which merelycover the mouth. These organs are modified inendless variety for various purposes,fromstrong jaws for cutting purposes to the long suc-torial tubes of the butterflies. The so-calleddental plates lining the crops of insects andCrustacea further comminute the food, and hairskeep the larger particles back until finely crushed.In the lobsters, crabs, etc., the "pinchers" (Fig.


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38 COMPAKATIVE DENTAL ANATOMY4) are but modifications of the limbs translatedfrom the locomotive series and set apart for spe-cial mouth organs. In the higher crustaceans thestomach is provided with calcareous plates orstomacholiths, with molar-like prominences forgrinding food by means of the powerful muscleswhich move them. These are interesting struc-tures, as they show how similar functions may

!

Fig. 4.X-ray of the claw of Lobster, modified, for dental purposes.develop analogous structures in dissimilar partswhich have no homology whatever.

In the Mollusca we find that the bivalves (theclams, oysters, mussels, etc.) are entirely withouthead or dental apparatus. The other groups,however, present some form of dental structurein most of their members. In the Cephalopodathe organs of mastication include a corneous beak,resembling that of a parrot, but reversed; withinthe oral opening there is a fleshy tongue, which


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THE TEETH OF INVEETEBEATES 39is armed with many transverse rows of recurved,spinous teeth. This is called the odontophore.This organ is controlled and moved by powerfulmuscles which draw it backward or forward, oreven protrude it, as in the snail. The teeth varygreatly in number in the many various species.Thus the nautilus may have but thirteen, and thesnail 12,000 to 40,000. As the teeth are worn offor lost, the ribbon-like tongue is uncoiled and new

Fig. 5. A, Radula of Winkle; B, Two rows of teeth, enlarged.teeth are brought into use. The upper part ofthe mouth is usually lined with a horny substance,against which the sharp-toothed tongue workswith a rasp-like motion (Fig. 5). The teeth varyin form, but are usually composed of a base, ashank or stem, and a cutting edge, the lattersimple or variously denticulated. The middlerow of teeth is called the rachidian; the lateralrows, the pleural teeth, and when an additionalrow occurs outside of this it is called the uncini.


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40 COMPAEATIVE DENTAL ANATOMYThe highest type of these molluscan teeth is calledthe Toxoglossal, or arrow-tooth, from its narrow,round form, often barbed, sometimes hollow toinject poison. They have but two rows,themiddle or rachidian being absent. The Bachi-glossa have only the middle row, or rachidianteeth. The teeth are small and varied in form,and prettily denticulated on the cutting edge.They are few in number. The Ptenoglossa,feather-toothed, is a small group. They lack themiddle rows, but have numerous small teeth onthe side of the tongue. The Docoglossa, chevron-toothed, is a large group, and presents consider-able variation among its members as to the pres-ence or absence of the different rows of teeth.The Eaphidoglossa, needle-toothed, have largenumbers of uncini teeth, the other rows varyingin different groups. They usually have a well-developed mandible, or jaw, which is hinged inthe middle. The Taenioglossa, bent-toothed, in-cludes the greater number of fresh-water snails.The teeth vary in number and are often absententirely. The common Helix, or air-breathingsnails, often present a pavement-like form andarrangement of the teeth, which are often of avery pretty pattern, or, again, are a mere hard-ened mass. The lingual ribbon, beset with suchteeth, is well adapted for filing off or rasping foodand drawing it backward into the mouth. Be-


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THE TEETH OF INVERTEBRATES 41sides that use it is also employed by some seamollusks for boring into shells to abstract thejuices of the animal vdthir


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CHAPTER IVTHE TEETH OF VERTEBRATES

In this great sub-kingdom true teeth are therule and not the exception. They are enderonicstructures, because they are derived from thedeeper portions of the derm, or corium of theintegument, and possess a calcified tissue calleddentin. This is the main tissue of the teeth inall Vertebrates, but in the higher forms the crownis covered and protected by a calcified epithelialtissue called enamel, and the root in the higherforms is surrounded by a calcified osseous tissueknown as cementum. The teeth of the lowerVertebrates, the Fishes and Reptiles, are com-posed mainly of dentin, and to this the othertissues, enamel and cementum, are added in thehigher forms.In position the teeth in the Vertebrates are

mostly confined to the oral cavity and to the bonesand cartilages of the head and face. In thehigher Vertebrates they are supported by the up-per and lower jaws,the maxillae and mandibleonly. In the lower forms they may extend to thethorax (or even to the oesophagus, as in some

42


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THE TEETH OF VERTEBRATES 43snakes), being supported by various bones andcartilages about the oral cavity. The palates andvomer (Fig. 6) often carry teeth in the fishes and

Fig.palates.

.Teeth of Pickerel (Esox lucius), showing teeth on vomer andsome calcified structures are on the gill arches.In the sawfish there is a special development ofthe premaxillary bones which carry teeth for de-fense only (Fig. 7).


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44 COMPARATIVE DENTAL ANATOMYThe Attachment of Teeth in vertebrates pre-

sents four varieties or methods which grade intoone another in different degrees. They are asfollows

1st. By means of a Fibrous membrane (as inthe Sharks and EaysFig. 8), in which the teethare imbedded, and which carries them up over theedge of the jaws. The teeth are brought up from

Fig. 7.Snout of Sawfish (Pristis pectinatus), showing teeth forripping open its prey.the floor of the mouth and rise up to replace thosewhich are lost from accident or use. These teetharise from the thecal fold.

2nd. By Elastic Hinge (as in many fishes, the


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THE TEETH OF VERTEBRATES 45Pike, Cod, etc.). The hinge is composed of strongfibrous ligament. Such teeth yield to pressure as

Fig. 8.Jaws of Sharkby fibrous membrane. (species unknown), showing teeth attachedthe prey passes over them, and then spring up tohold it while struggling. There are two types of


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46 COMPAKATIVE DENTAL ANATOMYElastic Hinge. In one type we find the tooth isattached to the pedestal of bone by means offibrons membrane which is located on the lingualside of the tooth and bony support. The base of

Fig. 9.Hinged tooth of Pike (species unknown). (After Tomes.)the tooth is attached to the bone by means ofelastic fibres. When the tooth is bent toward theoral cavity the elastic fibres pull it back to anupright position. If the fibres are cut the toothwill remain wherever placed, which shows there


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THE TEETH OF VEETEBEATES 47is no elasticity in the hinge. The other form pos-sesses no special elastic fibres but the hinge iselastic (Figs. 9 and 10).

3rd. By Ankylosis, when there is no interven-

Fig. 10.Hinged tooth of Hake (Merluccius). (After Tomes.)ing membrane, but the teeth and the jaw-bone areossified into one continuous piece like an anky-losed joint. Such teeth are sometimes but slightlyattached, or, again, so strongly as to bring awaya piece of bone when detached. Ankylosed teeth


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48 COMPARATIVE DENTAL ANATOMYare found in many fishes and reptiles. There arethree forms of ankylosed teeth : Acrodont, Pleuro-dont and Thecodont.In the acrodont tooth, a pedestal of bone de-

velops to support the tooth. During the processof development, the tooth is developed in thethecal fold and moved into position on the ridge

Fig. 11.Radiograph of head of Pickerel, showing acrodont ankylosis.(By Dr. E. H. Skinner.)of the jaw. After the tooth is well in position,the bone develops to support the tooth. Whenthe tooth is lost the bony support is lost also(Fig. 11).The pleurodont tooth is attached to the side of

the bone and may get directly above the ridge ofthe jaw. The top of the tooth projects above the


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THE TEETH OF VERTEBRATES 49jaw so as to enable the animal to use the tooth(Fig. 12).The thecodont tooth is ankylosed in a socketand is the type found in the higher reptiles and afew fishes. The tooth is more firmly supported inthis form than in any other (Fig. 13).

4th. By Implantation in a bony socket, as

Fig. 12.Mandible of Eel (Anguilla rostrata) , showing pleurodontattachment of teeth.

found in some Eeptiles and in the entire class ofMammalia. It is the method of attachment inman. There is an intervening membrane, a modi-fied periosteum, between the root of the tooth andits alveolus, and a special bone of attachment,called the alveolar process, which is raised uparound the root to support the tooth as it comesinto place, and is absorbed when it is lost (Fig.14).


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50 COMPAEATIVE DENTAL ANATOMY

Fig. 13.B e a k ofSawfish (Pristis pecti-na'tus) , showing theco-dont ankylosis.

There are three forms ofthe tooth which are attachedby gomphosis. The brachydonttooth, the hypsodont tooth andthe tooth of continuous growth.In the brachydont tooth, the

length of the root exceeds thatof the crown. The crown, veryearly in the life of the tooth,emerges from the gum. Inthose teeth we also find that theenamel covers the crown of thetooth and the cementum onlyextends to the gingival borderof the enamel. The teeth ofman is an example of this formof attachment. Fig. 14 alsoshows brachydont teeth.The hypsodont is character-

ized by the length of the crownbeing as great or greater thanthe root. The entire crowndoes not emerge from the socketand gums as one, but as thecrown is worn off the tooth con-tinues to erupt. The tooth is ofcontinuous eruption but not ofcontinuous growth. Figs. 15,65, 70 and 71 are examples of


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THE TEETH OF VEETEBEATES 51lrypsodont teeth. In most of these teeth we finda layer of cementum over the enamel.The tooth of continuous growth is one in whichthere is a persistent tooth germ and has beencalled a tooth with a "persistent pulp." Thereis a persistent pnlp but there is also a persistentenamel organ. In all of the teeth which are at-tached by gomphosis, there is a persistent ccmen-

Fig. 14.Radiograph of the mandible of a Wolf (Canis lupus), show-ing brachydont tooth attached by gomphosis.turn organ. With the tooth of continnons growththe dentin, enamel and cementum continue to de-velop and the tooth continues to emerge from thesocket. A great many of the animals which haveteeth of continuous growth have but one set ofteeth. All of the rodents have some teeth whichare of continuous growth. Fig. 16 shows theincisor of a squirrel.Tooth Forms. The forms of the teeth in the

Vertebrates present great variety. In the lowestclasses, the Fishes and Eeptiles, the simple conical


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52 COMPARATIVE DENTAL ANATOMY


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THE TEETH OE VERTEBRATES 53form predominates, as the teeth in these lowtypes are modifications of the simple cone. Thisis the primitive typal form of tooth from whichall later and complex forms were derived. Thereis considerable variation of the cone, however, inthese classes, some fishes, as the Kays, etc., havingplates or a pavement of teeth of flattened shape.Others are of cylindrical or prism-like outline, bntthe majority of fishes and reptiles exhibit modi-

Fig. 16.Roentgenogram of the mandible of a Squirrel (Sciurus niger),showing the incisor of continuous growth and brachydont molars and pre-molars.fications of the simple cone, which is employed forprehension only.In the Mammalia there is a greater variety and

more complex forms of teeth. These are formedby evolution of the primitive typal cone, byduplication and modification of the cone to formbicuspid, tritubercular, quadritubercular, etc.,forms, as of the molars. Teeth are developedfrom the primitive cone for various functionsthus the incisors are molded from a single cone


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54 COMPARATIVE DENTAL ANATOMYby flattening of its base, to cut substances; thecanine is a single cone elongated and sharpened,to seize and tear flesh; the premolars (bicuspids),as in man, are formed by the addition of a secondlingual cone to the primitive buccal cone, to crushfood, or by the addition of a third cone to formthe tritubercular molar, or of the fourth to formthe quadritubercular molar, etc., to grind food.There is sometimes special development of specialteeth for secondary purposes, as of the incisorsof the Elephant, Sirenia, Narwhal; or the caninesof the Walrus, the extinct carnivora, the wildboar; or of the blades of the premolars of thecarnivora. The premolar and molar teeth wereevidently developed by the duplication of conesby fusion or addition, which are traceable backalong the paths of their development throughgeologic ages, to simple conic reptilian forms.There was first the simple cone alternating withthat of the opposite jaw, as in the living reptiles(Haplodont form) ; then the double cone, formedby the addition of a second cone (as the premolarsof man) ; then the third cone was added to formthe triconodont type, which was modified to thetritubercular form of molar (the primitive typeof all molars) ; then the projecting heel or cingule(talon) led to the formation of the fourth tubercle(the quadritubercular molar) ; then the additionof the fifth and other tubercles formed the addi-


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THE TEETH OF VERTEBRATES 55tional types. Thus the cones were duplicatedand tubercles added to form the multitubercu-lar types. These molar tubercles are rounded(Bunodont), as in Man, the Bears, Mastodon, etc.;raised to form cutting blades in the carnivora ; orfolded and duplicated (Lophodont) in the herbiv-orous mammalia to form broad triturating sur-faces, etc.The Number of the Teeth in Vertebrates varies

greatly in different classes, and may even vary inthe same genera. Some fishes are entirely with-out teeth of any kind; others have but one (as theMyxine and other parasitic forms), which is usedas a lancet to cut the flesh for the purpose ofdrawing blood; others have a few teeth, and thenumber increases up to the thousands, in the bonyfishes, and which may stud the mouth in everyconceivable position. These are of continuoussuccession, so that the numbers are always indefi-nite. The reptiles have fewer teeth than thefishes, but these succeed one another continuously,and the exact number cannot always be deter-mined. Different individuals of the same specieswill present great variations. In the mammalsthe number can be determined with greater preci-sion, as each species, especially of the higherforms, has a definite number. In the lower mam-mals, as with the reptiles, the number is somewhatindefinite, but with the advance in the scale it be-


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56 COMPAEATIVE DENTAL ANATOMYcomes more exact. Some species are devoid ofteeth entirely, as in some Ant-eaters. Othershave but one tooth, as the Narwhal. A Dolphinhas but two; the Elephant has but two incisors,and but four molars in use at one time. Somerodents have but two incisors and four molars ineach jaw; the Sloths have but eighteen teeth;Man, the old world Monkeys, and some otherMammals have but thirty-two teeth, etc.The number increases in various families to an

excessive degree: thus some of the Armadilloshave ninety-eight ; some Whales sixty; the com-mon Porpoise eighty to ninety; the GangeticDolphin one hundred and twenty, and the trueDolphin one hundred to two hundred.While there is thus great variation in the num-

bers of teeth in the various classes of Vertebratesand even among the members of the same generaand families, there is a rule governing all whichrenders their study intelligible. This is based ona scientific classification and arrangement bymeans of which all teeth and tooth forms can beproperly understood.

Vertebrate teeth are classified into various di-visions having references to their forms, position,and functions. In the fishes and reptiles the teethare adapted mainly for seizing and tearing, andconsequently are undifferentiated as to positionand function. There is little variety in different


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THE TEETH OF VEETEBEATES 57parts of the jaw as to the forms of the teeth, butonly as to size, and there is not sufficient differ-entiation to admit of classification. But onefunction is performed in these low classes, thatof prehension, for mastication is not yet devel-oped. In the higher Vertebrates, the Mammalia,however, the teeth are more differentiated andspecial forms are evolved for special uses. Thusthe teeth situated in the front of the oral cavity,from their form, are called incisors, or cutters,and their function is to cut or divide food. Thelarge conical teeth situated immediately distallyof the incisors are called the canines (from beingextra well developed in the dog and other carniv-orous animals), and are used for seizing and tear-ing flesh. The next teeth are the molars, thecrushers and grinders, which perform the func-tion of mastication and insalivation. These aredivided into two classes, the premolars and truemolars. The premolars are the permanent teethjust distally of the canines which succeed the de-ciduous molars. These are called the bicuspidsin man. After these are the full or true molars,which are the true grinding teeth. Thus the per-manent teeth of mammals are classified into fourgroups, (1) the Incisors, (2) the Canines, (3)the Premolars, (4) the Molars. With this ar-rangement it is convenient to express in a mathe-matical scheme the number of the teeth of any


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58 COMPARATIVE DENTAL. ANATOMYmammal by means of what is called the DentalFormula. In this scheme the teeth are repre-sented by numbers in the form of fractions,those of the upper jaw being the numerator andthose of the lower jaw the denominator. Thusthe dental formula of Man is, for the permanentteeth,

reading that he has on each side of each jaw 2 in-cisors, 1 canine, 2 premolars (or bicuspids), and3 molars,the initial letter of each class beingused for abbreviation. The teeth of mammalsmay be expressed in the same way. The decidu-ous teeth of Man have the formula,

i. H c. H m. H = 20,2-2 1-1 2-2there being no premolars or bicuspids in the de-ciduous series. The formula of the Elephant is,

. i_i o-o 6-6 oai. c. m. = 26,0-0 0-0 6-6or of the Eat,-


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THE TEETH OF VERTEBRATES 59i.e., the first incisor to the left of mesial line andabove fraction line. The same tooth below wouldbe ifTT The right upper first molar 1m\, the lowerleft second molar |m2 , the right upper second pre-molar 2 pm |, the left lower third molar |m3 ; thenumber being distal of the initial letter and theline mesial.The classes of the teeth in Mammals.The Incisors take their name from the office

they perform in the function of mastication,i.e.,to incise, to cut, but the term is applied to theupper teeth located in the premaxillary bone an-terior to the intermaxillary suture, whatever theirform, in mammals. The teeth in the mandiblewhich occlude with the upper teeth (in the pre-maxillary bones) are also incisors. Thus thetusks of the elephant are incisors, although theircutting function is completely aborted and theseteeth are employed as tools and weapons only.The function of cutting or dividing food is per-formed by various organs throughout the animalkingdom. Teeth for this purpose are developedvery low in the scale of life, as the cephalopodshave cutting teeth; the sea-urchin has highlyspecialized incisors ; the insects and worms cut bymeans of the mandibles, jaws, etc. The beaks ofturtles and birds are employed as cutting imple-ments, but these are not true teeth. The fishesand reptiles have no true incisors, as the teeth are


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60 COMPARATIVE DENTAL ANATOMYof simple conical shape and are employed for pre-hension only. The "sheep-head" possess an-terior teeth which resemble the incisors of a sheepand ronnd crushing teeth in the posterior region.Most of the lower mammals are deficient in regardto cutting teeth, the teeth being all of the molartype for grinding. The higher forms possesswell-marked typical incisors. Thus the incisorsof the Herbivora are well developed for cuttingpurposes. In the Camivora the incisors are re-duced, for the cutting function is usurped by thelong blades of the sectorial premolars and molars.In the Marsupials, Insectivora, Rodentia, Cheirop-tera, and others the incisors are specially devel-oped for special purposes. In the Quadrumanathey foretell the form of these teeth in man, whichthey resemble, in whom incisors are formed bythe modification of the single cone,the base be-ing flattened to form a cutting edge.The Canines. This tooth is the first succeedingthe incisors, and is immediately distal to the inter-maxillary suture. It is implanted in the maxil-lary bone proper above, and is probably modifiedfrom the premolar series. It is called the Caninefrom being extra well developed in the dog andother carnivorous animals. It is the principalprehensile tooth in mammals, and is thereforefirst in function, though second in position in thedental series. It is implanted by a single root,


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THE TEETH OF VERTEBRATES 61usually, and is modified from a single cone. Itis the most primitive type of tooth, being" nearestto the cone shape as found in the fishes and rep-tiles in all parts of the jaws. In the mammals itstill preserves the primitive form, though modi-fied variously in different classes. The lowermammals have no canines, but the Dolphin andCetacea have conical canine-like teeth in all posi-tions like the Reptiles. In the Marsupials it be-gins to assume specialized forms. It is absent inthe Proboscidce and Rodentia and in some of theRuminants. In some of the Herbivora it is ofincisor-like form, and is ranged with these teethfor cutting purposes. It is excessively developedin the Musk-deer, Boar, Walrus, and other ani-mals, for battle or other secondary purposes.But it is in the Carnivora that the canine attainsits greatest glory. In its monstrous developmentin some fossil carnivores it extended far beyondthe lower jaw, and was of a saber-like form whichis recalled in lesser degree in the extinct CaveTiger and the Lion and Tiger of today. In theFelidcB these teeth are long, curved, and piercing,for tearing flesh and destroying life. In theCanidcB they are reduced in size, and are roundand stout. In the Baboon they are still of largesize, though reduced in the Monkeys and Apes,and are lowered to the level of the other teeth inMan. In man the canines are reduced in form


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62 COMPARATIVE DENTAL ANATOMYand size, but still retain suggestions of the fea-tures of these teeth in the Carnivora.The Tubercular and grinding teeth. The very

lowest form in which grinding organs appear arethe cusped prominences on the triturating platesof the Crustacea and some insects, but these arenot true teeth. They are, however, analogous totrue grinding teeth, as they are employed for thesame purpose. Very few of the Invertebratespossess triturating apparatus of any sort. In theVertebrates, crushing teeth appear in some formsof fishes, which have well-developed pavementteeth of various forms for crushing the shells ofmollusks and crustaceans. These are not truemolars, however, as they do not triturate food norinsalivate it. Tuberculate teeth proper do notappear until in the higher forms of reptiles, assome of the lizards have teeth, which are slightlytubercular, and these are the beginnings and fore-runners of the molar series in the Mammalia.The lizards show the first tendency to the dupli-cation of cusps, which are repeated over and overin various directions in the Mammalia. In theMammalia the molar series of the permanentteeth is divided into two sections, (a) the pre-molar (or succedaneous teeth), and (b) the truemolars, which have no deciduous predecessors.The grinding teeth of the deciduous set aremolars. The premolars are the small grinders


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THE TEETH OF VERTEBRATES 63which are found between the canines and truemolars among mammals, and vary in number indifferent species. They are the principal crush-ing members of the dental series, and are placedmidway of the cutting and grinding teeth proper.In man the upper premolars (or bicuspids) are ofsimple form, being composed of two or threecones united together. The lower premolars varyfrom this form. In the higher Apes the pre-molars are of the same form as in man, but arecoarser and larger. In the lower Quadrumanathey are reduced to simple crushing teeth with theouter cusp enlarged and the inner reduced. Inthe Carnivora they are highly specialized on ac-count of the tubercles being raised into large cut-ting blades. In the Herbivora they are similarto the true molars in form, and are developed fortriturating purposes. In the Insectivora they arevery variable in shape, but possess, like the truemolars, long, sharp cusps for crushing insect cov-erings. In the Rodentia they are entirely absentin most of the species of this extensive order, asthere is a large vacant space between the incisorsand the grinding teeth. The true molars arefound alone in the Mammalia, and are highlyspecialized teeth, being developed for the per-formance of the function of mastication. In theBruta the teeth are all molars of a simple form,for crushing purposes. Some low forms have


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64 COMPAKATIVE DENTAL ANATOMYflat tooth-shaped plates of horn which answer thepurpose of grinding teeth. In some of the ant-eaters, the tongue and roof of the mouth arearmed with horny plates for crushing. In the In-sectivora the molars are highly developed, withmany long, pointed cusps for crushing the hardcoverings of insects. In man, the molars are ofsimple tuberculate form of a lower grade of or-ganization. His molar teeth are indeed of thetype of the early Eocene mammals. In theCarnivora the molars are reduced in size andnumber, and the premolars are highly developed.In the more omnivorous species the number ofmolars is increased, and they have rounded tuber-cles for grinding a mixed diet. In the Herbivorathe molars are highly developed for the mastica-tion of an extreme diet, with pleatings and fold-ings of the dental tissue which insure a constantlyrough face for the difficult reduction of resistingvegetable fiber. In the Quadrumana the molarsare similar to these teeth in man, being simplytuberculate for a mixed diet.The molar teeth of the Mammalia are classified

as follows, according to shapeHaplodont. The crown undivided or simple

(as in the single teeth of the Getacea, Carnivora,Rodentia, etc.).Ptychodont. The crown folded on the sides (as

in the Rodentia molars).


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THE TEETH OF VERTEBRATES 65

o o o oVMMso-Q-^ o-c^-o^

*%vFig. 17.Phyletic History of the Molar Cusps. (After Osborn.)

a, The Reptilian stage. (Haplodont.)b, Early Mammalian stage. (Triconodont.c, Triangular stage. (Tritubercular molar.)d, Quadritubercular molar.


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66 COMPARATIVE DENTAL ANATOMYBunodont. Crown supporting tubercles (as in

Man, Carnivora, Mastodon, etc.).Lophodont. Summit of crown with transverseor longitudinal folds (as in Herbivora).The Evolution of Teeth. The molar teeth pre-

sent great varieties among mammals, but all havebeen derived in some manner from the primitivecone. All investigators are not agreed as to theexact manner of the evolution of the molars. Instudying the evolution of the teeth, we are forcedto gain our knowledge from three sources; viz.,anatomy, embryology and paleontology. Eachone of these sciences cast some light upon theevolution of the teeth and these sciences all go toprove that the teeth have evolved from the simplecone.From the study of paleontology, different in-

vestigators do not agree as to the exact mannerof the evolution of the molars. All agree thatthe molars have started from a single cone, butthey do not agree as to the exact detail. Fromthe study of the teeth of fossils, it seems to thewriter that it may be possible that all of the molarteeth did not evolve in the same manner, but havebeen the result of radiation. By radiation in bi-ology is meant that the plant or animals of a cer-tain class all begin from a common center andbranch off in different directions until we havemany plants that resemble each other but very


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THE TEETH OF VERTEBRATES 67little, and each group must be traced back to theoriginal center in order to show their relationbetween the different groups. In the study of theteeth, it seems as if radiation has played an im-portant part. Beginning with the primitive cone,we have three theories for the evolution of themolar, each of which has probably played a partin the animal kingdom. Some of the animalshave probably had their molar teeth follow oneplan of evolution and some have followed another.The Cingulum Theory. This is one of the old-

est, if not the oldest theory of tooth evolution.It is based on the fact that all primitive teethshow a tendency to form a ridge that developsnear the gingival margin of the tooth, or at theneck, which is a ridge of enamel on the teeth ofthe higher mammals. The cingulum theory isthat upon this ridge developed small cusps orcingule which increased in size in later genera-tions and resulted in the extra cusps. Thestrongest evidence in favor of this theory is thatwe find a great many cingule on the teeth of fos-sils and upon the teeth of some of the modernanimals. There is very little proof that themolars of modern mammals are evolved in thismanner.The Concrescence Theory. This theory is

based on the fact that the reptiles and fishes, pos-sessed a great many teeth and it has been proven


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68 COMPAEATIVE DENTAL ANATOMYvery conclusively that the mammals have evolvedfrom the reptiles. The concrescence theory isthat the large number of teeth which we find inthe fish and reptiles have fused together and madea number of large teeth with a varying number ofcusps. There is very little to support this theoryexcept a few fossil forms that have been found invarious parts of the country. The teeth of themastodon and mammoth seem to be made up ofa number of cusps fused together. The plates ofthe elephant's teeth might also come under thisform of evolution, but there is nothing to provethat the majority of mammals have had theirmolars evolved in that manner. Briefly, theargument against the concrescence theory is thatthe number of teeth have decreased from thefishes to reptiles and we find the primitive reptileshaving single conical teeth. The majority of theteeth of the fishes were located on the palates andvomer and in the higher reptiles and mammalsthe maxillae and mandible carry the teeth. Thelow forms of mammals generally have few teethand they are all conical as in the ant-eaters andarmadillos. As we reach the higher forms wefind that the cusps begin to appear on these coni-cal teeth.The Tritubercular Theory. The tritubercular

theory is that the molars of mammals haveevolved from a molar with three cusps. This


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THE TEETH OE VERTEBKATES 69theory as first advocated has been modified someby various investigators nntil it may be brieflystated as follows: Eealizing that all teeth havedeveloped from the primitive conical teeth whichwe find in the reptiles by the addition of othercones, this first or primitive cone is called theprotocone in the upper arch and the protoconid inthe lower. In the evolution of the premolars,such as we find in the upper teeth of man, theprotocone becomes the lingual cusp (some writersclaim the protocone becomes the buccal cusp inthe upper premolars but there is little fo supportthat theory), and the second cusp becomes thebuccal cusp and is called the deuterocone. In thelower premolars the protoconid becomes the buc-cal cusp and the deuteroconid becomes the lingualcusp. Some of the premolars, often the lowersecond premolar of man, are three cusp teeth,made so by the addition of the third cusp which iscalled the tritoconid, and when present in theupper arch is called the tritocone. In some of themammals, a fourth cusp is found which is calledthe tetarocone in the upper and the tetaroconid inthe lower. In the carnivora, we find the cusps ofthe premolars arranged in a row antero-pos-teriorly.

In the evolution of the true molars, there isadded to the protocone a small anterior cuspwhich is called the paracone and a small posterior


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70 COMPAEATIVE DENTAL ANATOMYcusp which is called the metacone, in the upperarch. In fact the upper cusps are distinguished

Fig. 18.Occlusal view of teeth of Opossum (Didelphis virginiana),showing triangular molar crowns.by the endingthe ending ' ' conid. '

and the lower cusps byThis gives three cusps


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THE TEETH OF VERTEBRATES 71in an antero-posterior line, forming a three-cusped crown called the triconodont form. Thisis the type of the early forms of the mammalianmolar teeth and is still preserved in some of thecarnivora, seals, lemurs and other living species.The next stage is the shifting of the cusps so as toalter their relative positions to form a triangle.In the upper arch the protocone shifts to the

Fig. 19.Diagram showing the occlusion of the trigon and trigonid.a. Protocone (Lingual cusp).b. Paracone (Mesio-buccal cusp).c. Metacone (Disto-buccal cusp).

1. Protoconid (Buccal cusp).2. Paraconid (Mesio-lingual cusp).3. Metaconid (Disto-lingual cusp).

lingual and becomes the mesio-lingual cusp, leav-ing the paracone and the metacone on the buccalside. The paracone is the mesio-buccal cusp andthe metacone the disto-buccal cusp. This is thetritubercular molar crown of early geologicaltimes from which all other molars of the presentmammals are probably developed. This triangu-lar molar crown is still present in the opossum(Fig. 18), some insectivora, lemurs, and others.


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72 COMPARATIVE DENTAL ANATOMYThis triangular arrangement of the cusps formsthe trigon of the upper molars.In the lower molar, the primitive cusp is calledthe protoconid and moves to the buccal side andbecomes the mesio-buccal cusp. The paraconidbecomes the mesio-lingual cusp and the metaconidbecomes the disto-lingual cusp, thereby formingthe trigonid of the lower molar. Thus the tri-

Fig. 20.Diagram showing the occlusion of the talonid of the lowermolar in the trigon of the upper.1. Protoconid (Mesio-buccal cusp). a. Protocone (Lingual cusp).2. Paraconid (Mesio-lingual cusp). b. Paracone (Mesio-buccal cusp).3. Metaconid (Disto-lingual cusp). c. Metacone (Disto-buccal cusp).4. Hypoconid (Disto-buccal cusp).

angles of the upper and lower molars alternate(Fig. 19),the apex of the upper one being di-rected lingually and the lower one buccally,sothey pass in a shear-like motion. The next stepis the addition to the trigonid (of the lowermolar), on the disto-buccal surface, of a heel ortalonid, which may support one, two or threecusps. The buccal is called the hypoconid, thedisto-buccal the hypoconulid, and the disto-lingual


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THE TEETH OF VEETEB-EATES 73the entoconid. The position of the talonid is suchthat the hypoconid falls in the center of the trigonof the npper molar (Fig. 20). There is then de-veloped on the disto-lingnal surface of the trigon(upper molar) a talon which carries a cusp calledthe liypocone, thus making a quadritubercularmolar as seen in the upper molars of man. Thisfourth cusp falls between the lower cusps as

Fig. 21.Diagram showing the position of the talon of the upper molar.1. Protoeonid (Mesio-buccal cusp). a. Protoeone (Mesio-lingual cusp).2. Paraconid (Mesio-lingual cusp). b. Paracone (Mesio-buccal cusp).3. Metaconid (Disto-lingual cusp). c. Metacone (Disto-buccal cusp).4. Hypoconid (Disto-buccal cusp). d. Hypocone (Disto-lingual cusp).

shown in Fig. 21. The cusps of the lower molarcontinue to develop and change. The paraconidbecomes reduced in size and is missing in thelower permanent molars of man, but exists in thedeciduous lower first molar as a small cusp. Theparaconid can also be seen in the lower teeth ofthe opossum. The position of the paraconid andthe cusps of the talonid, as seen in the lower firstdeciduous molar of man, is shown in Fig. 22 with


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74 COMPARATIVE DENTAL ANATOMYthe cusps named. The lower first deciduousmolar shows the gradual reduction in size of theparaconid better than any other tooth of man.In the lower first permanent molar of man, the

paraconid is absent and the talonid carries threecusps,the hypoconid, the hypoconulid and theentoconid. The hypoconid is the buccal cusp andfalls in the center of the talon of the upper molar

FiG. 22.Diagram to show the paraconid of the lower first deciduousmolar in the relation of the talonid and trigon of the upper molar.1. Protoconid. a. Protocone.2. Paraconid. b. Paracone.3. Metaconid. c. Metacone.4. Hypoconid. d. Hypocone.5. Hypoconulid.

(Fig. 23). In the four-cusp molar of man, asseen in the lower second molar, there is somequestion as to which cusp is lost or fails to de-velop. These forms are further complicated insome of the mammals, especially in the herbivorahowever, the history of the molar cusps can betraced with considerable accuracy through thevarious steps of their evolution from the earlygeological times to the present day.


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THE TEETH OF VERTEBRATES 75Evolution of Occlusion. The teeth have been

evolved from a single cone to the more complextypes and likewise the arrangement of the teethhave changed. In the fishes we find only an oc-clusion of tooth-bearing surfaces. There may beteeth on the maxillae and mandible which occludewith each other as a mass. There may be teethon the palates and vomer which occlude with

Fig. 23.Diagram to show the relation of the upper first and secondmolars and the lower first and second molars with the cusps named.1. Protoconid.3. Metaconid.4. Hypoconid.5. Hypoconulid.6. Entoconulid.

a. Protocone.b. Paracone.c. Metacone.d. Hypocone.

masses of calcified substances on the tongue.This arrangement is called occlusion of tooth-bearing surfaces, an example of which is seen inthe myliobatis (Fig. 24).The next step is the occlusion of rows of teeth.

In the non-poisonous snakes, as the python (Fig.25), we find two rows of teeth above and one be-low. The lower row occludes between the upper


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/D COMPAKATIVE DENTAL ANATOMYrows. One of the upper rows is situated on themaxillae and the other on the palates. In the alii-

Fig. 24.Myliobatis, showing occlusion of tooth-bearing surfaces.gators and crocodiles there is one row above andone below, but no occlusion of the individual teeth.

Fig. 25.Skull of Alligator (Alligator mississippiensis), showing oc-clusion of rows of teeth.

The teeth of some of the primitive birds were alsoarranged this way (Fig. 25).We next find occlusion of individual teeth, onwhich there has been no development of extra


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THE TEETH OF VERTEBRATES 77cusps. Such an example is seen in the armadilloand the teeth may be said to represent the originalprotocone and protoconid (Fig. 26).

Fig. 26.Skull of Armadillo (Euphractus minutus) , showing occlu-sion of rows of teeth.In the occlusion of cusps, which is the type of

occlusion found in higher mammals, we find theteeth have evolved so as to present a tooth with

Fig. 27.Skull of Mountain Lion, showing occlusion of cusps.many cusps which is so arranged as to occludewith the cusps of the opposing tooth. This givesa tooth which has a great amount of grinding sur-face (Fig. 27),


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78 COMPAKATIVE DENTAL ANATOMYEvolution of the Temporo-mandibular Articu-

lation. With the evolution of the tooth formsand the occlusion of the teeth we notice a cor-responding change in the jaw forms and thetemporo-mandibular articulation. In those ani-mals which have occlusion of tooth-bearing sur-faces, the temporo-mandibular articulation is veryloosely constructed and the condyle and glenoidfossa have very little shape. In those animalswhich have occlusion of rows of teeth, there ismore of a condyle and glenoid fossa. In the alli-gators, owing to the anatomical structure of theanimal, we find that the condyle is concave andthe glenoid fossa convex and the mandible is themore stationary bone when the mouth is opened.In those animals with occlusion of the cusps, wefind the articulation influenced by the shape of thecusp. In the animals which have long cusps,the glenoid fossa is decidedly concave, and inthose in which the cusp is short the glenoid fossais only slightly concave and often entirely flat.When we find the cusps of the teeth so arrangedthat the tooth has a long antero-posterior diam-eter the condyle has a great bucco-lingual diam-eter. The greatest diameter of the tooth isalways the opposite to the greatest diameter ofthe condyle. In man, we find that the shapeof the condyle changes as the occlusion changes.In fact, in young animals of whatever family, the


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THE TEETH OF VERTEBRATES 79temporo-mandibular articulation does not assumea definite shape until after the teeth are in occlu-sion.The Succession of the Teeth in Vertebrates.

The Fishes and Reptiles have the teeth suppliedin endless succession. When teeth are lost, oth-ers rise to take their places by various processes.In the Sharks and Bays, the teeth are supportedby a fibrous membrane which carries them upover the edge of the jaw (Fig. 28), bringing newones into place as the old teeth are lost. Hingedteeth are replaced as lost by new ones which areconstantly developed at the base of the old teeth.Ankylosed teeth are replaced by the successiveteeth being raised up beside or in the place of theold teeth, and new bone of attachment is repro-duced to support them. Socket teeth are replacedby the advance of new teeth from within the bodyof the jaw when the old teeth are lost, as in man.In the Fishes and Eeptiles many sets of teeth aredeveloped during the life of the individual. Inthe higher forms the teeth arise de novo from themucous membrane. The number is constant andindefinite.In the present Mammalia there is generally but

two sets of teeth produced. The Elephant hassix molars, each one of which may be taken as aset of teeth, as but one erupts at a time and nevermore than two are present at one time. Mam-


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80 COMPAEATIVE DENTAL ANATOMY

Fig. 28.Roentgenogram of jaw of Shark, showing endless successionof teeth. (By Dr. E. H. Skinner.)


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THE TEETH OF VERTEBRATES 81

Fig. 28-A.Roentgenograms of jaws of Parrot-fish (Scaridce), show-ing continuous succession of teeth. (By Dr. E. H. Skinner-. ) .


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82 COMPAEATIVE DENTAL ANATOMYmals are divided into two classes in reference tothe possession of one or two sets: those havingbut one set are called Monophyodont, those giv-

FiG. 29.Teeth of human. Diphyodont and heterodont.ing rise to two sets are called Diphyodont. Themonophyodont mammals produce but one set,which persists through life,as some Armadillos,Sloths, Cetacea, the common Eat, and others.


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THE TEETH OF VERTEBRATES 83The first set remains permanently, is more or lesscontinually growing, and is never displaced by asecond set. The diphyodont mammals have firsta deciduous set which supplies the dental wantsof the individual during the first years of exist-ence. It is then shed and replaced by the per-manent set, which should remain during the life-time of the individual. The teeth which succeedthe deciduous teeth are called succedaneous teeth,but additional teeth are sometimes producedwhich have no deciduous predecessors. Thus inman the ten anterior teeth are shed and replacedby permanent teeth, but there are also six addi-tional teeth, the true molars, in each jaw whicherupt without displacing any deciduous teeth.The monophyodonts are usually homodonts,

i.e., the teeth are all alike and of the same formin all parts of the jaw. The diphyodonts areheterodonts,i.e., the teeth are of various anddifferent forms in various parts of the jaw,asthe incisors, canines, premolars, and molars ofthe Mammalia. So as a rule homodonts developbut one set of teeth, and heterodonts two sets, al-though there are noted exceptions to this rule.

In mammals the deciduous set arises de novofrom the mucous membrane, and the permanentteeth are given off from the dental lamina. Asa rule each tooth of the first set is displaced ver-tically by a similar tooth of the permanent set.


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84 COMPARATIVE DENTAL ANATOMYThere are examples, however, of milk teetli whichhave no successors, as some Rodent incisors, andof permanent teeth which have no deciduous pred-ecessors, as the true molars. Some Rodentsas the rathave hut one set and are monophyo-dont. The elephant's molars succeed each othercontinuously from behind by lateral displacement.The Carnivora have a well-developed milk set.The Herbivora have also a milk set, but the truemolars arise de novo. In the Quadrumana thetwo sets are similar to those of man.


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CHAPTER VTHE TEETH OF FISHES

The first and lowest class of the Vertebrates isthat of the Fishes. They are entirely aquatic intheir habits, and breathe by means of gills. Thebody is bare, or covered with horny scales. Thelimbs are modified into organs for swimming.The nervous system is centralized in the spinalcord, but there is little enlargement of the cephalicportion. In the lowest forms, as the Amphyoxus,there is no enlargement at all. The osseousstructures are soft and cartilaginous, and partakeof the general low structure of the class. Thevertebrae are cupped before and behind, except inthe Pike and some other forms which show a steptoward the reptilian character of being cupped infront and balled behind. The jaws are often mov-able and loose, and capable of protrusion and re-trusion.The Teeth of Fishes. The true fishes have trueteeth. There are many kinds of teeth among the

many species of fishes, which are developed inevery conceivable position upon the various bonesand cartilages of the head. They are sometimes

85


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86 COMPARATIVE DENTAL ANATOMYproduced in countless numbers, as in the Salmonand Pike, or may be reduced to only one tooth, asin the Myxine, or there may be none at all.There is no general rule as to the number, posi-tion, or form in this great class. They are verygeneral in form and undifferentiated, and mayvary extensively even among the individuals ofthe same species.Forms and Function. The teeth of fishes are

derived from the simple cone, by various modi-fications. The conical teeth present great vari-ety,from long, fine, hair-like forms, to short,stout cylinders, and in others they may vary fromcone to cylinder, from cylinder to plate. Theteeth of fishes are designed for prehensile pur-poses mainly, as there are no masticating teethproper in this class, though some species havepavement-like teeth, for crushing purposes. Theconical teeth are often fine or hair-like; or soshort as to be only felt with the fingers; or longand slender; or long and strong; or stout andshort,such teeth being attenuated cones inshape. The cone may merge into the cylinder,this into the compressed triangular form (as inthe Shark), or stout with rounded summits (asin the Wolf-fish), or flattened plates (as in theEay) or incisor-like form (as in the Sargus).The arrangement is very general and indefinite.

Structure. The teeth of fishes are generally


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THE TEETH OF FISHES 87composed of vaso-dentin,a soft dentin with acirculation,the point of the tooth only being cov-ered with enamel in some forms. This dentin issomewhat denser than the bone of the jaw, butis not so dense as the dentin of the higher Verte-brates. In some forms it is not covered, and inothers, as the Sharks, it is protected by a shinyenamel-like substance, but this is not true enamel.In other forms, as in the Sargus, the dentin isyet harder, and is covered with a thick layer ofdense substance developed by a distinct organ.Sometimes this enamel is covered by a layer ofcementum. In others again there is a mere os-seous substance which results from the calcifica-tion of the pulp.The Development and Succession of the Teeth

in Fishes. In the great majority of fishes thegerms of the teeth are developed directly from themucous membrane of the mouth, throughout thev hole period of succession. This is peculiar tothe class. In all species the teeth are shed andconstantly renewed throughout the whole life ofthe individual. Generally there is more than onesuccessional tooth developing, so that severalteeth are in process of formation destined to suc-ceed one another in regular order. The mode ofsuccession varies with the method of attachment,however, of which there are several in this greatclass,i.e., ankylosis, elastic hinge, and fibrous


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88 COMPARATIVE DENTAL. ANATOMYmembrane. In ankylosed teeth, the most commonform is for the new tooth to rise up nnder theold tooth and by absorption displace it; othersdisplace the old tooth from the side by absorbingits supporting cone. Hinged teeth are developedin the folds of the mucous membrane, and as theold teeth are lost through use, new ones rise upto take their places. In the fishes having fibrousmembrane attachment, the teeth are developed inthe thecal fold on the inner side of the base ofthe jaw, and are perfected in growth as they arecarried upward over the edge of the jaws by themembrane in which they are imbedded. Thus inthe sharks the whole phalanx of the numerousteeth is ever marching slowly forward in rotaryprogress over the occlusal borders of the jaws,the teeth being successively cast off after havingreached the outer margin and fulfilled for a longeror shorter period their appointed function, andare succeeded by others that rise up continuallyto take the places of those that are lost.

Descriptive. In the lowest forms of the car-tilaginous fishes, as the Myxines, Lampreys, andother parasitic types, we find some destitute oftrue calcified teeth, but instead possessed of ahorny structure of a conical sharp-pointed andoften slightly recurved form. Some, as the Hag-fish, have a lance-like tooth for piercing the vic-tim to suck the blood. In others, horny teeth may


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THE TEETH OF FISHES 89be developed upon the roof of the mouth or palate,or upon the tongue, or may be supported by pecul-iarly developed labial cartilages.The Eiasmobranchii (the Sharks, Rays, etc.)

have the body coveredwhen covered at allbytooth-like structures which are small and closeset, and the layer is called Shagreen. Whenlarger and more scattered they form dermalplates or tubercles, and when they take the form

Fig. 30.Teeth of Shark.of spines are called dermal defenses. These cov-erings constitute the "placoid exo-skeleton,"which in minute structure precisely resemblesteeth. The protruding surfaces are frequentlyornamented with an elegant sculpturing of vari-ous patterns. There are no true jaws, but in-stead cartilaginous forms of the palato-quadratearch and of Meckel's cartilage. The teeth ofsharks are always very numerous and arranged


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90 COMPAEATIVE DENTAL ANATOMYin concentric rows on the inside and summits ofthe mandible below and the palato-quadratearches above (Fig. 30). They are imbedded inand borne upon the fibrous membrane whichlines the inner surfaces of the arches, and bywhich they are carried forward over the edge ofthe jaws. The teeth lie flat against the membraneuntil brought to the edge, where the turningbrings them upright for use. Thus the first rowstands upright on the margin of the jaw and doesservice until lost, then those of the next row riseup to take the place of those lost, and the succeed-ing rows follow in regular orders,the successionbeing endless. The teeth are developed from thebottom of a longitudinal fold of the lining mem-brane of the mouth at the lower edge of the jaws,called the thecal fold, from which they are con-tinually carried up over the edge of the jaws. Inall species the teeth are largest in the front orcenter of the jaws and diminish in size backward.The teeth of sharks are peculiarly adapted to thedestructive habits of their possessors. The sim-plest form is that of a simple cone with a sharppoint on a broad base, or it may be a triangularcone. In most of the sharks, however, the teethare flattened into a triangular form, sometimeswith serrated edges. The serrations are oftendeep and sharp and the tooth makes a very ef-fective implement for cutting and tearing. The


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92 COMPAEATIVE DENTAL ANATOMYeral form of the teeth of sharks is that of a sub-compressed triangle, but some forms have smallaccessory tubercles on each side of the centralblade. Sometimes the blade is wide, short, andstout ; again it is long and slender, or oval or leaf-shaped; the flat faces are sometimes smooth, andagain grooved or wrinkled.The Sharks merge into the Uays, in forms

which have both cutting and crushing teeth. Theteeth of all Eays are in pavement-like arrange-ment of various shapes, and are formed for crush-ing purposes. They are quite contact so as to forma more or less continuous sheath over the wholesurface of the jaw. The shapes of the plate-liketeeth are quite various, and sometimes presentbeautiful forms, in both recent and fossil species.Some are perfectly oval; some quadrangular(which fit closely together) ; in someas Mylio-batisthe teeth are of horny or bony structure,and are closely joined together; in some forms theteeth are rounded eminences ; in all varieties theyare formed in a thecal fold imbedded in a fibrousmembrane, are carried up over the edge of thejaw, and are replaced as lost, just as in theSharks.The Saw-fish is a Ray, and is remarkable for

the unique elongated snout, which is covered onboth edges with true teeth, set in bony socketsand growing from persistent pulps. This fish


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THE TEETH OF FISHES 93sometimes attains an enormous size, with a snoutsix feet long and twelve inches wide (see Fig. 13).The Teleostei, or true bony fishes, possess teethwhich are ankylosed to the various bones of thehead; some are hinged, and but rarely are theyimplanted in sockets. They consist usually of

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