Chelonia, an order of reptiles including the various forms of tortoise and turtle. Their most distinctive character is the more or less complete inclosure of the body by a dorsal and a ventral shield, of which the former is in part due to a modification of the vertebral spines and of the ribs. Within these shields the head, limbs, and tail can be more or less retracted. The absence of teeth is also very characteristic. The Chelonia include marine, fresh-water, and terrestrial forms; the known living species number about 260, the majority occurring in warm countries; they are represented by numerous fossil forms from the Upper Jurassic onwards. The large family Testudinidae, with oval, horny-plated dorsal shield, includes both terrestrial and marsh forms, such as Testudo—e.g. the Greek tortoise, Terrapin, Emys, Chelys; a second small family, Trionychidae, with oval, softer shield, includes river forms, such as the well-known snapping-turtle (Trionyx ferrox); a third family, Chelonidae, with heart-shaped shield, includes five species of marine turtles—e.g.
Chelone viridis. The order will be chiefly discussed under the titles TORTOISE and TURTLE.