FRESH-WATER OR MARSH TORTOISES
The family of turtles or tortoises (Emydidae) represented at the present time by the common terrapin, painted tortoise, and box tortoise of the United States, and commonly called fresh-water turtles or tortoises, comprises the largest group of living chelonians—nearly a third of all existing members of the order. They are widely distributed over all parts of the earth except Australia, and are of very varied habits. Some are almost exclusively aquatic; others, like the painted tortoise, are partially so, while others, especially the common box tortoise, are almost as exclusively terrestrial as the true land tortoises, dying even, if forced to live long in water.
The shell in the more aquatic forms is depressed or flattened, but in the terrestrial kinds may be as highly arched as in the true land tortoises. The feet are adapted primarily for walking, but nearly always have the toes webbed, and the digits are longer than are those of the land tortoises. Only the two or three middle toes have claws. Some species have developed hinges in the plastron, whereby they may be completely closed up after the head and legs are withdrawn within the shell. Most of the species are carnivorous in habit, but a few, like the box tortoise, are strictly vegetarian.
Geologically the fresh-water tortoises have a not very ancient history, going back no farther than do the land tortoises, that is, to the beginning of the Cenozoic or Age of Mammals. Fully fifty species are known from the Tertiary rocks of North America, or more than three-fourths as many as are now living upon the earth.
The family at most can be said to be only amphibious in habit, and has no striking aquatic adaptations, since the shell is well developed and is covered with horny shields. The flattened shell of the more aquatic forms is characteristic, as is also the greater degree of webbing between the toes.