32tt 
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, w T hich cannot be classi¬ 
fied with any of the four living orders. Such are the 
Ichthyosaurus , Plesiosaurus , Pterodactyle , Megalosaurus , 
and Iguanodon. 
