52 Vertebrata. 



tapering snouts, in which the longest teeth of the lower 

 jaw notch the sides of the upper jaw. The alligators 

 of the New World have heads oval or rounded in 

 front, and in all of them the lower jaw teeth are 

 hidden by the edge of the upper, when the mouth is 

 closed. The gavial of the Ganges has a long, slender- 

 pointed head, and is the smallest of the group. 



Of all the reptiles the crocodiles are those which 

 in point of structure approach most closely to the 

 birds. They have a gizzard-like stomach, a nictitat- 

 ing membrane in the eye, an immovable joint between 

 the tibia or leg bone and the first bone of the tarsus 

 or ankle, a single carotid or neck-artery, and many 

 other structural peculiarities which show their super- 

 iority over other reptiles. Among the orders of the 

 reptile class now extinct, there was one which in- 

 cluded bipedal forms which had possibly a kangaroo- 

 like mode of progression, and one of flying reptiles, 

 which indicated a still closer relationship to the birds. 



CHAPTER XL 



CLASS IV. AVES (BIRDS). 



29. General Characters. These familiar verte- 

 brates are characterised by possessing an epidermal 

 clothing of feathers, warm blood, a four-chambered 

 heart, no teeth, and in general an adaptation for aerial 

 locomotion. The mode of progression on the earth 



