OF DIGESTION. 



105 



food ; some using them merely for seizing and securing their 

 prey, as the lizards, frogs, crocodiles, and the great majority 

 of fishes. In such animals, the teeth are nearly all alike in 

 form and structure, as for instance, in the alligator, (Fig. 71,) 

 the porpoises, and many fishes. A few of the latter, some of 



Fig. 70. 



Fig. 71. Fig. 72. 



the Rays, for example, have a sort of bony pavement, (Fig. 

 72,) composed of a peculiar kind of teeth, with which they 

 crush the shells of the mollusks and crabs on which they 

 feed. 



218. The Mammals, however, are almost the only verte- 

 brates which can properly be said to masticate their food. 

 Their teeth are well 

 developed, and pre- 

 sent great diversity 

 in form, arrangement 

 and mode of inser- 

 tion. Three kinds 

 of teeth are usually 

 distinguished in most 

 of these animals, 

 whatever may be Fig. 73. 



their mode of life ; namely, the cutting teeth, incisors ; the 



a 



