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a reptile, member of the order Chelonia; most are aquatic or semiaquatic, fresh water or marine, but lay eggs on land. They have webbed feet or flippers and their body is covered by a horny shell from which only the legs, head and neck, and tail protrude when needed. The upper shell is called the carapace and the undershell the plastron.
There are inconsistencies in terminology. In the USA 'turtle' is used broadly for all reptiles with a shell, 'terrapin' applies to a large family, Emydidae, and 'tortoise' refers to the slow moving terrestrial species (the land turtles) that enter water only to drink or soak. In Great Britain and Australia 'tortoise' is applied generally to all members of the group except the marine species with paddle-shaped limbs which are called 'turtles'. 'Terrapin' is often used to describe the young tortoises commonly sold as aquarium pets.
Includes genera Chelonia, e.g. C. mydas (green turtle), Testudo, e.g. T. graeca (spur-thighed tortoise), T. hermanni (Hermann's tortoise), Gopherus, e.g. G. agassizi (desert tortoise), Clemmys, e.g. C. marmorata (Pacific pond turtle), Chrysemys, e.g. C. picta (painted turtle), Platemys, e.g. P. platycephala (Bolivian side-neck turtle).