Tortoises. 49 



in the act of swallowing, so that all the material of 

 the egg may be saved for food. Snakes are rare in 

 cold and more abundant in warm climates ; they are 

 also more numerous in continental than in insular 

 regions. 



CHAPTER X. 



TORTOISES AND CROCODILES. 



27. Order 3, Chelonia (Tortoises). This order 

 consists of those reptiles whose bodies are enclosed 

 in a bony case composed of a dorsal or upper convex 

 shield, called the carapace and a flat ventral or under 

 shield, the plastron. The carapace is notched in 

 front and behind, and between it and the plastron 

 project the head and neck, the limbs, and the tail. 

 These parts can be retracted under cover of the bony 

 case. Each shield consists of a layer of epidermis or 

 tortoise-shell, and a layer of bone, which in the cara- 

 pace consists of dermal plates added to the tops of 

 the spines of the vertebrae, the surfaces of the ribb ; 

 and a row of marginal bony plates below these. The 

 plastron also consists of nine plates of ossified dermis 

 covered by a symmetrical series of horny laminae. 

 The skull is short, rounded, and not armed with 

 teeth, which are replaced by horny beak-like jaws. 

 The lower jaw is in one piece in the adult. The 

 shoulder-girdle consists of three bony rods, two in 



E 



