312 COMPARATIVE ZOOLOGY. 
are firmly united, and furnished with numerous teeth im- 
planted in distinct sockets. The lower jaw extends back 
of the cranium. The heart has four cavities, but the pul- 
monary artery and aorta communicate with each other, so 
that there is a mixture of venous and arterial blood. They 
have external ear-openings, closed by a flap of the skin, 
and eyes with movable lids; a muscular gizzard; a long, 
compressed tail; and four legs, with feet more or less web- 
bed, and having five toes in front and four behind. The 
existing species are confined to tropical rivers, and are car- 
nivorous. The eggs are covered with a hard shell. 
There are three representative forms: the Gavial of the 
Fig. 293.—Alligator (A. Mississippiensis). Southern States. 
Ganges, remarkable for its long snout and uniform teeth ; 
the Crocodile of the Nile, 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 closed; and the 
Alligator of the Mississippi, whose canines, 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 medieval ages of geological history, the class of 
Reptiles was far more abundantly represented than now. 
Among the many forms which geologists have unearthed 
