Turtle, (order Testudines), any reptile with a body encased in a bony shell, including tortoises. Although numerous animals, from invertebrates to mammals, have evolved shells, none has an architecture like that of turtles. The turtle shell has a top (carapace) and a bottom (plastron). The carapace and plastron are bony structures that usually join one another along each side of the body, creating a rigid skeletal box.
There are approximately 356 species of turtles living on land in all continents except Antarctica and in both salt water and fresh water. Tortoises (family Testudinidae) live exclusively on land and have anatomic features distinguishing them from other turtles, but the term tortoise has long been used to refer to other terrestrial testudines as well, such as the box turtle and the wood turtle. Similarly, terrapin was sometimes used to describe any aquatic turtle but is now largely restricted to the edible diamondback terrapin (Malaclemys terrapin) of the eastern United States.
