CHELONIA, or TESTUDI NATA, tes-du-di-na'ta, an order or sub-class of Reptilia (q.v.) containing the turtles and tortoises, and one of the most sharply defined groups of vertebrates. The body is enclosed in a more or less well-developed bony box, from which the head, neck, tail and two pairs of limbs protrude, and into which they may be completely retracted in some tortoises and terrapins. This box is a unique feature among reptiles• and is divided into a dorsal shield or carapace composed of numerous der mal bones usually arranged regularly and sup ported on the ribs and vertebral spines, and a ventral shield or plastron likewise composed of dermal bones, among which are incorporated the clavicles and interclavicle. Both carapace and plastron are usually encased in horny plates, which do not correspond to the bony pieces. Except for the head, neck and tail, which are flexible, the vertebrae are immovably ankylosed with the carapace. The skull is very compact, with a small brain cavity, and ex hibits many structural peculiarities, of which two of the most apparent are the complete fixation of the quadrate bone and the substitu tion of horny cutting plates for teeth on the margins of the jaws. Both shoulder and pelvic girdles are permanently enclosed within the shell, and the latter may be firmly united to both carapace and plastron. The limbs may be flattened paddles in the sea turtles, or true walking legs with free toes in the other groups. All chelonians have well-developed lungs; but some aquatic forms have additional respiratory organs in the form of vascular areas and papil be in the pharynx and cesophagus, or respira tory sacs in connection with the rectum. Pul monary respiration is maintained partly by the movements of the neck and limbs in and out of the shell and partly by the hyoid apparatus. Without any known exceptions, chelonians are oviparous, and, after burying the eggs in the earth, allow them to be hatched by the warmth of the sun. Except in the colder regions tur
tles are found throughout the world, and are mostly inhabitants of fresh water, though a considerable number are terrestrial and a few marine. Between 200 and 300 living, and a great many extinct, species are known, many of the latter belonging to families not represented in the recent fauna. The chelonia are divided into four sub-orders: 1. Atheca, in which the carapace is of leathery consistency and is supported by nu merous small, irregular, separate bones free from the ribs. There is one family (Derma tochelydide), containing the leather-turtle (q.v.).
2. Trionychoidea, with a true carapace com posed of dermal bones united to the ribs, but covered by a leathery skin,. and lacking the marginal bones; the pelvis is free from the plastron. There is one family (Trionychidw), the soft-shelled turtles. See TURTLES.
3. Cryptodira, in which the carapace is cov ered with horny plates and provided with marginal bones, and the pelvis is free from the plastron. It embraces the following families: Chelonidw, including marine turtles, as the green, loggerhead and tortoise-shell turtles; Testudinidw, land-tortoises and gopher tortoise; Chelydridee, snapping-turtles and alligator turtles; Cinosternidw, terrapins and mud turtles; and several others confined to the tropics of both hemispheres.
4. Pleurodira, in which the carapace is simi lar to that of the Cryptodira, but the pelvis is ankylosed to the plastron, and the neck folds in a horizontal instead of a vertical plane. They are all inhabitants of fresh waters in the tropical parts of both the Old and New Worlds. Representative families are the Pelomedusidee, tartaruga (q.v.) and Chelydida', matamata (q.v.).
Consult Gadow, 'Amphibia and Reptiles' Natural History,' London 1901), and Baur, in a bulletin of the United States National Museum.