=Tortoise, Turtle, Terrapin.=—The turtles belong to the order of reptiles called _chelonians_. No one 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. [Illustration: FIG. 271.—SNAPPING TURTLE (_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 Its head and tail are very large and cannot be withdrawn into the shell.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, according to Lyddeker, can trot cheerfully along with three full-grown men on its back. “Tortoise shell” used for combs and other articles is obtained from the overlapping scales of the _hawkbill turtle_, common in the West Indies. The _diamond-back terrapin_, found along the Atlantic Coast from Massachusetts to Texas, is prized for making soup. [Illustration: FIG. 272.—A RATTLESNAKE. ] [Illustration: FIG. 273 _a_.—HEAD OF VIPER, showing typical triangular shape of head of venomous snake. ] [Illustration: FIG. 273 _b_.—SIDE VIEW, showing poison fangs; also tongue (forked, harmless). ] [Illustration: FIG. 274.—VIPER’S HEAD, showing poison sac at base of fangs. ] [Illustration: FIG. 275.—SKULL, showing teeth, fangs, and quadrate bone to which lower jaw is joined. See Fig. 284. ]