. HOW ANIMALS EAT 269 



appear (as in the salmon) on the upper maxillary. As 

 to mode of attachment, the teeth are generally anchy- 

 losed (fastened by bony matter) to the bones which sup- 

 port them, or simply bound by ligaments, as in the shark. 

 In a few fishes, the teeth consist of flexible cartilage ; 

 but almost invariably they are composed of some kind 

 of dentine, enamel and cement being absent. 



Of amphibians and reptiles, toads, turtles, and tor- 

 toises are toothless ; frogs have teeth in the upper jaw 

 only; snakes have a more 

 complete set; but saurians 

 possess the most perfect 

 dentition. The number is 

 not fixed even in the same 

 species; in the alligator it 

 varies from 72 to 88. The 



teeth are limited tO the FIG. 231. -Poison Apparatus of the Rattle- 



snake: g, gland, with duct, leading to the 



jawbones in the higher fang,// *, elevator muscles of the jaw, 

 r , \ i which, in contracting, compress the gland; 



lOrniS (Saurians); but in s , salivary glands on the edge of the jaws; 



others, as the serpents, w > nostril - 

 they are planted also in the roof of the mouth. With 

 few exceptions, they are conical and curved (Fig. 224). 

 In the serpents they are longest and sharpest; and 

 the venomous species have two or more fangs in the 

 upper jaw. These fangs contain a canal, through which 

 the poison is forced by muscles which compress the 

 gland. The bones to which they are attached are mov- 

 able, and the fangs ordinarily lie flat upon the gums, but 

 are brought into a vertical position in the act of striking. 

 As a rule, the teeth of reptiles are simply soldered to 

 the bone which supports them, or lodged in a groove ; 

 but those of crocodiles are set in sockets. Reptilian 

 teeth are made of dentine and a thin layer of cement, 

 to which is added in most saurians a coat of enamel on 

 the crown. 



