TURTLE

any member of the Chelonia, an ancient order of reptiles chiefly characterized by a shell that encloses the vital organs of the body and more or less protects the head and limbs. Although there has been much confusion over the scientific as well as the common name of the group, most scientists now accept the term Chelonia rather than Testudines or Testudinata. Two common names are in wide use: tortoise and turtle. Tortoise is applied in the British Isles to all members of the group except the few marine species, all of which have paddle-shaped limbs. Turtle has long been much more broadly applied in the United States, with the addition of terrapin for some edible species. Usage both in the British Isles and in the United States has left the group without a general name comparable to bird or mammal. The American Society of Ichthyologists and Herpetologists standardized the common names of the reptiles found in the United States, assigning turtle to all of those with a shell. The name tortoise is employed secondarily for the slow-moving terrestrial species, primarily those of the genera Testudo and Gopherus. (order Chelonia), reptile chiefly characterized by a protective shell that encloses the vital organs of the body. Turtles are toothless, generally slow-moving and unaggressive animals that range in length from less than 10 cm (4 inches) to more than 2 m (6.5 feet). They have sturdy, sprawling limbs with short feet or (in marine turtles) paddlelike flippers, and they have a protective bony shell overlaid with horny shields. The shell is separated into an upper section, the carapace, joined at the sides to a lower section, the plastron. Turtles are found in most parts of the world and have existed in relatively the same form for the last 200 million years. Most of the 200 to 250 living species are aquatic or semiaquatic, and the majority are found in or near the tropics, but many range into temperate regions. Some turtles are terrestrial, and a few are marine, with the rest living in freshwater. Turtles feed on soft plant material or small animals or both. They are able to fast for long periods. Breeding usually takes place once yearly; the female lays her whitish, rounded or elongated eggs on land, usually in a hole she has dug with her hind legs. Turtles have provided humans with meat, eggs, and tortoiseshell. Some species of turtles are kept as pets. Nonmarine turtles are commonly called tortoises in Great Britain, while in the United States some edible turtles are known as terrapins. The world's living species of turtles are placed in two suborders. In the members of one, Pleurodira, the neck is bent sideways to draw the head into the shell, and in those of the other, Cryptodira, the head and neck are withdrawn backward into the shell. The pleurodires are now restricted in range to South America, Africa, Madagascar, Australia, New Guinea, and adjacent islands. The cryptodires, by contrast, are found on all the continents except Australia and include about four-fifths of all living species of turtles. The largest family among the cryptodires is that of the common freshwater turtles (Emydidae), which includes about one-third of all living species and has a geographic range equal to that of the suborder as a whole. These turtles, many of which occur in the eastern half of the United States, are mostly aquatic or semiaquatic in habitat. Next in number of species are the true tortoises (family Testudinidae), with about half as many species as are found in Emydidae. The slow, plodding tortoises of fable belong to this widely distributed terrestrial group, the gigantic species of which are confined to the Galpagos and other oceanic islands. (See tortoise.) The other families of cryptodires include mud and musk turtles (family Kinosternidae); sea turtles (family Cheloniidae), which are found in all the warmer oceanic waters of the world; and snapping turtles (family Chelydridae), which are fairly large and aggressive turtles common in North America. The suborder Pleurodira, with its members now restricted in range to the continents of the Southern Hemisphere, includes about 20 percent of all living species of turtles. The two (living) families in it are the snake-necked turtles (family Chelidae), so named for their long head and neck; and the side-necked turtles (family Pelomedusidae), from which the common name of the suborder as a whole is derived.