Page:First course in biology (IA firstcourseinbio00bailrich).pdf/380

 can have any difficulty in knowing a member of this order. The subdivision of the order into families is not so easy, however, and the popular attempts to classify chelonians as turtles, tortoises, and terrapins have not been entirely successful. Species with a vaulted shell and imperfectly webbed toes and strictly terrestrial habits are called tortoises. Species with flattened shells and strictly aquatic habits should be called terrapins (e.g. mud terrapin). They have three instead of two joints in the middle toe of each foot. The term turtle may be applied to species which are partly terrestrial and partly aquatic (e.g. snapping turtle (Fig. 271)). Usage, however, is by no means uniform.

(Chelydra serpentina).

Most reptiles eat animal food; green terrapins and some land tortoises eat vegetable food. Would you judge that carnivorous chelonians catch very active prey?

The fierce snapping turtle, found in ponds and streams, sometimes has a body three feet long. Its head and tail are very large and cannot be withdrawn into the shell. It is carnivorous and has great strength of jaw. It has been known to snap a large stick in two. The box tortoise is yellowish brown with blotches of yellow, and like its close kinsman, the pond turtle of Europe (Fig. 266), withdraws itself and closes its shell completely. Both lids of the plastron are movable, a peculiarity belonging to these two species. The giant tortoise of the Galapagos Islands, ac-