CLASS OF REPTILES. 



357 



Fig. 358. 



is but rarely found. Some have three eyelids, in others 

 they are wholly wanting, as in the serpent ; to this may be 

 ascribed its fixed look. 



The auditory apparatus is much less complete than in 

 mammals, or even in birds. The external ear is almost always 

 completely wanting; there is no 

 auditory canal, and the drum of the 

 ear is on a level with the outer 

 surface of the head; the tympanic 

 cavity is imperfectly formed, and 

 seems to be a sort of dependence of 

 the pharynx. The bones of the 

 ear most frequently are wanting, and the cochlea is often 

 rudimentary ; the organs of smell are but little developed ; 

 the tongue is sometimes thick and fleshy, but generally thin, 

 dry, and protractile ; in serpents (Fig. 358) and lizards it is 

 bifid : in the chameleon the tongue becomes an instrument of 

 prehension, for it can be darted from the mouth to the distance 

 of several inches, and thus flies and other small insects are 

 caught by means of a viscous ball which terminates it. 



463. Few reptiles live entirely on vegetable food ; they 

 are almost all carnivorous, and pursue a living prey, which 

 they swallow entire. The mouth is almost always large in 



Fig. 359. Head of the Crotalus, or American 

 Eattlesnake. 



the cleft, and is so dilatable in serpents as to enable them to 

 swallow animals having a larger diameter than themselves. 

 The two branches of the lower jaw (mi, Fig. 359) are united 

 only by ligament, and the tympanic bone (t), and the 

 mastoid bone (ma), by means of which the lower jaw is 

 articulated to the cranium, are both moveable, and thus the 



