364 GENERAL ZOOLOGY 



tirely on land. These tortoises have a very convex upper 

 shell ; the lower portion is provided with a transverse hinge, 

 which makes it possible for the animal to bring the two 

 parts of the shell closely together, thus forming a box, 

 within which are concealed head, neck, legs, and tail. A 

 remarkable sex dimorphism occurs; the eyes of the male 

 are red, those of the female, brown. More flattened species, 

 like the painted tortoise (Chrys'emys), are better adapted to 



Fig. 189. Photograph of an alligator. (Greatly reduced) 



an aquatic existence. The snapping turtles {Chely'dra) 

 have extremely powerful jaws. In some localities both the 

 snapper and the soft-shelled turtle (Am'yda) are used as 

 food. 



Alligators and Crocodiles. The large reptiles known as alli- 

 gators and crocodiles belong to the Crocodiria (Lat. croco- 

 dilus, "crocodile"). The heart in the crocodilians is highly 

 specialized, having four separate chambers by the complete 

 separation of the ventricle into two chambers. Alligators 

 (Fig. 189) differ from crocodiles in having the canine teeth 

 of the lower jaw fitting into pits in the upper jaw ; in the 

 crocodiles they fit into notches in the side of the jaw. A 

 species of each kind is found in Florida, though both have 

 been sought so eagerly for the teeth and skin that they are 

 now found only in the more inaccessible places. Crocodil- 



