Turtles, tortoises, and terrapins are all turtles. The term "tortoise" is used for terrestrial turtles with high-domed shells and elephantine hindlimbs, whereas the term "terrapin" is used properly for some highly aquatic turtles (genus Malaclemys) of eastern North America, although it frequently is used in error for American box turtles in the genus Terrapene. Turtles are easily recognized and distinguished from all other vertebrates by their shells. Shells are composed of a dorsal carapace and a ventral plastron. These are usually rigidly connected on the sides by bridges. Shells are composed of bony plates that form within the skin. These are fused to underlying vertebrae and ribs. Most shells have a covering of horny plates made of keratin, a protein which, in other vertebrates, forms scales, hair, nails, claws, or horns. In some turtles, the plates of bone and keratin are reduced or absent, and the shell is covered by leathery skin. Many turtles have one or more hinges in their shells, usually in the plastron. These allow the shell to completely enclose the withdrawn head, limbs, and tail. The plastron of males in many species is indented to accommodate the female's shell during mating.
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