CLASS REPTILES. 313 



plastron below. The head, tail, and legs of the creature 

 emerge from between these structures. Because of this 

 heavy armor the turtles are ungainly animals ; but equally 

 because of it they can well defy most enemies by merely 

 withdrawing the exposed parts, and remaining quiet. 



The water forms are usually known as turtles; those 

 that live on land as tortoises. Some of the sea-turtles 

 are large. The great leather-backed turtle is said to 

 reach a weight of half a ton. The green turtle of the 

 Atlantic coast is much used in the making of soups. 

 The large hawk-bill turtle of the tropical seas has an 



FIG. 128. Head of the American alligator (Alligator mississipiensis) . From Eckstein. 



outer covering which furnishes the "tortoise-shell" of 

 commerce. The snapping turtles are fresh-water types 

 and are quite ferocious in disposition. Fairly common 

 are the box-tortoise, the painted turtle, the soft-shelled 

 turtle, and the speckled tortoise. 



The Crocodilia (Fig. 128) are aquatic, living in the 

 swamps and sluggish waters at the margins of the great 

 rivers. There are some twenty species, two of which are 

 found in the southern United States. Others are found 

 in the Nile, the Ganges, and the other great tropical 

 rivers. The alligator is smaller and has a somewhat 

 broader snout than the crocodile. 



