The order Chelonia is divided into two suborders: the Cryptodira, comprising species
                        that pull their heads into the shell by a telescoping movement of the neck; and the
                        Pleurodira, comprising freshwater species that hide their heads by bending the neck
                        sideways. About 250 species are grouped into 9 families. Found only in the
                        temperate or tropical zones, most species are adapted to a freshwater or terrestrial
                        habitat, and a small group to sea life. 

                        In American usage the term tortoise
                        refers only to species of a terrestrial, or
                        land-inhabiting, family, belonging to the
                        Cryptodira; all other members of the order
                        are called turtles. In Great Britain the
                        term turtle is restricted to sea turtles of
                        the families Cheloniidae and
                        Dermochelyidae. 

                        Physical Characteristics

                        Turtles show a wide variation in size, from the 15-cm
                        (6-in) box turtle of North America to the giant
                        sea-dwelling leatherback, which may attain a length of
                        about 2.4 m (about 8 ft). The upper shell of the turtle,
                        under which the head, limbs, and tail can be more or
                        less completely withdrawn, is called the carapace. The
                        lower shell, characteristically flat, is called the plastron.
                        The two-part shell is connected to the vertebrae and
                        ribs. The structure and size of the carapace and
                        plastron vary among turtles, along with adaptive
                        changes in behavior and mode of life. 

                        A turtle shell typically consists of two layers: an inner
                        bony layer, the sections of which are called plates; and
                        an overlapping, horny layer, made up of so-called
                        shields. Although it is hard, and in some species quite
                        thick, the turtle shell is a surprisingly sensitive
                        structure because of the many nerves embedded in it. The horny outer layer of
                        some turtles?especially the hawksbill turtle?has been used in making ornamental
                        objects; this material is popularly called tortoiseshell. 

                        The bones of the turtle skull are immovably connected; the animals have no teeth,
                        although in an embryonic soft-shelled turtle, traces of them have been detected. The
                        turtle also lacks a breastbone. The heart, as in other reptiles except crocodilians,
                        has three chambers, but it operates almost as if it had four because of the presence
                        of an incomplete partition in the ventricle. 

                        Behavior

                        Although few species of turtles can be
                        considered strictly carnivorous or
                        herbivorous, some inland habitats prefer
                        vegetation, and aquatic species are
                        mostly carnivorous. All turtles lay eggs,
                        which they bury in holes. They are
                        generally long-lived; some species live for
                        more than 100 years. 

                        Freshwater and Wood Turtles

                        The freshwater and wood turtles, of the family Emydidae, constitute about half the
                        species in the order. These turtles are characterized by their fully or partly webbed
                        feet and their usually flat, streamlined shells; both features help in diving and
                        underwater escape. Several genera in this family, however, have evolved to terrestrial
                        life and show an arched carapace, more typical of tortoises. Among these is the
                        American box turtle. Box turtles are further protected by a hinged lower shield that
                        affords complete closure. Other important U.S. species of this family include the
                        terrapins, especially the diamondback; the cooters in the genus Pseudemys; the
                        map turtles in Graptemys; the painted turtles in Chrysemys; and the wood and pond
                        turtles in Clemmys. 

                        Soft-Shelled Turtles

                        The flattened, round-shelled turtles of the
                        family Trionychidae are known commonly
                        as soft-shelled turtles, although in fact
                        only the marginal bones of the carapace
                        are replaced by soft parts. By virtue of
                        their thin-edged, flat shell these turtles
                        are well adapted to a life of concealment
                        on the bottoms of lakes and rivers. Tough
                        skin, instead of the usual horny layer,
                        covers the shell bones, giving the turtles
                        a leathery appearance. The family is
                        represented in the U.S. by the genus
                        Trionyx, which includes two species and several subspecies. Soft-shelled turtles are
                        known for their aggressive behavior, and the bite of a large specimen is capable of
                        severing a human finger. 

                        Snapping Turtles

                        In the snapping turtles of the American family Chelydridae, the plastron consists
                        merely of two narrow, crossed bands. As with the soft-shelled turtles, large, sharp
                        jaws compensate for the vulnerability of the exposed underside. Examples of these
                        turtles include the common snapping turtle and the alligator snapping turtle, one of
                        the largest freshwater turtles in the world; an adult alligator snapping turtle may
                        reach a weight of 90 kg (200 lb). 

                        Sea Turtles

                        The best-known marine turtles are the green, loggerhead, hawksbill, and ridley
                        turtles of the family Cheloniidae and the huge leatherback turtle of the family
                        Dermochelyidae. The leatherback is the largest living turtle, sometimes reaching a
                        weight of about 540kg (1200 lb). Sea turtles live mostly in warm waters around the
                        world and are graceful swimmers, with limbs modified into long flippers that enable
                        them to migrate long distances. The green turtle, for example, migrates from the
                        coast of Brazil to breed on the small island of Ascension, some 3340 km (1400 mi)
                        out in the Atlantic. 

                        When female sea turtles crawl above the tide line to bury their eggs, they and their
                        large egg clutches are easy prey to shoreline predators and especially to turtle
                        hunters, who prize the meat of the green turtle. Also, because of diminishing
                        nesting grounds due to shore development and the drowning of turtles in fishing
                        trawls, the green turtle and the Pacific ridley are close to extinction, and the
                        leatherback is considered an endangered species. 

                        Tortoises

                        The reptiles known as tortoises are
                        generally restricted to the terrestrial
                        family Testudinidae. The carapace of
                        tortoises is high and domelike. The upper
                        and lower shields of some tortoises, such
                        as those of the African genus Kinyxis, are
                        equipped with a hinge, which permits
                        complete closure. The giant tortoises of
                        the Galápagos Islands belong in this
                        group. 

                        Because their shielding and their heavily
                        scaled limbs afford complete protection, tortoises generally have mild dispositions.
                        The forelimbs of burrowing tortoises such as the American gopher tortoise, are
                        especially adapted for digging deep underground burrows used as shelters against
                        uncomfortable extremes in temperature.