CLASS OF EEPTILES. 



349 



the hot-blooded vertebrata, and exhibit analogies with the 

 other classes. 



456. The cranium is always small, and the face elon- 

 gated. As in birds, the lower jaw is composed of several 

 distinct pieces, and is articulated as in them with the os 

 quadratum or tympanic bone. Even this bone is sometimes 

 suspended to a moveable lever, as in serpent?, by which the 

 dilatability of the mouth is greatly increased. The upper 

 jaw is in general immovable, but is so articulated in serpents 

 as to perform some movements. In several, as in lizards and 

 tortoises, the bones of the cranium are extended over the tem- 

 poral spaces like a buckler, thus causing the head to appear 

 large. Finally, the head is generally not very moveable, and 

 is articulated with the column by a single condyle with 

 several faeettes. 



Fig. 345. Common Land Tortoise. 



Fig. 346. Egyptian Aspic. 



457. In lizards and crocodiles, and other reptiles formed 

 after the same way, there are in general but few anomalies ; 

 the ribs are more numerous than in birds and mammals, and 

 protect the abdomen as well as the chest. Serpents have 

 neither sternum nor limbs, and the free extremities of the 

 ribs seem to be used as organs of locomotion. In the cou- 



