324 



COMPARATIVE ZOOLOGY. 



skin, and eyes with movable lids; a muscular gizzard; a 

 long, compressed tail; and four legs, with feet more or 

 less webbed, and having five toes in front and four be- 

 hind. The existing species are confined to tropical rivers, 

 and are carnivorous. The eggs are covered with a hard 

 shell. 



There are three representative forms : the Gavial of the 

 Ganges, remarkable for its long snout and uniform teeth; 

 the Crocodiles, mainly of the Old World, whose teeth are 

 unequal, and the lower canines fit into a notch in the edge 

 of the upper jaw, so that it is visible when the mouth is 



Fig. 303.— Alligator (A. Mississippiensis). Southern States. 



closed ; and the Alligators of the New World, whose ca- 

 nines, in shutting the mouth, are concealed in a pit in the 

 upper jaw. The toes of the Gavials and Crocodiles are 

 webbed to the tip ; those of the Alligators are not more 

 than half-webbed. 



In the mediaeval ages of geological history, the class of 

 Reptiles was far more abundantly represented than now. 

 Among the many forms which geologists have unearthed 

 are numerous gigantic Saurians, which cannot be classi- 

 fied with any of the four living orders. Such are the 

 Ichthyosaurus, Plesiosaurus, Pterodaetyle, Megalosaurus, 

 and Iguanodon. 



