298 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



teeth, (2) the canines (tusks of carnivores, eye teeth of 

 man), (3) premolars, or deciduous molars or bicuspids of 

 man, and (4) the permanent or true molars. The func- 

 tion of the first is the cutting off of morsels of food, of 

 the second is seizing and holding the prey. The jaw teeth 

 — i. e., the molars and premolars — are the true masticat- 

 ing teeth. This grouping of the teeth into kinds having 

 different functions is very characteristic of mammals. 



4. Variation of the Teeth. — The teeth of different or- 

 ders and families of mammals vary in relative size and 

 relative number of the kinds, and in the structure of the 

 molars. 



(a) Relative Size. — In man all the teeth are of similar 

 size, forming thus a continuous even arch; but in the 

 more specialized mammals some of the teeth may be 

 enormously developed — for example, the canines in car- 

 nivores, and especially the walrus; the incisors in ro- 

 dents, and especially in proboscidians (elephants). In 

 cases of enormous development, such as the incisors of 

 rodents and the tusks of the walrus, the boar, and the 

 elephant, the pulp is permanent, and the tooth grows 

 continuously. Such teeth have a hollow at the base. 

 Thus, teeth are sometimes of definite and sometimes of 

 indefinite groiuth. 



(b) Number of Teeth and Relative Number of the Kinds. 

 — The normal number of mammalian teeth seems to be 

 forty-four, and any less number must be regarded as the 

 result of gradual loss; precisely as in the case of toes 

 of less number than the normal five. But the whole 

 number, and especially the relative number of the sev- 

 eral kinds, vary in a way which is very characteristic 

 of the different orders and families of mammals. This 

 introduces the subject of the dental formula, which is a 

 compendious way of expressing the number of different 

 kinds of teeth in mammals, very necessary in descrip- 



