Turtle, reptile (order Chelonia) with a shell that almost encloses the body. A land turtle is sometimes called a tortoise, and some freshwater turtles are given the name terrapin. The turtle has existed for more than 175 million years, and there are now about 230 species spread over the warmer parts of the world, 44 of them in the United States. Some species are becoming rare because they are hunted for their flesh or for their shells. The turtle's shell is made of two parts—the upper carapace and the lower plastron. The vertebrae and ribs are fused to the carapace. The shell is covered with horny plates, except in the soft-shell and the leathery turtle, where it has a leathery covering. There are five kinds of marine turtles: the leathery turtle and the green, hawksbill, loggerhead, and ridley turtles. Except for the last, all are found in North American waters. All come ashore to lay their eggs in sandy beaches: Freshly hatched turtles have to rush down to the shore after hatching to avoid predators. The green turtle is the species most used for turtle soup, and the hawksbill provides tortoise shells. The land and freshwater turtles are smaller than the marine turtles. They range from 2-in (5-cm) mud turtles to the 2-ft (61-cm) 100-lb (45-kg) alligator snapping turtle or “stinkpot”, the reptilian equivalent of the skunk. The box turtles have a hinged plastron so that they can completely shut themselves in their shells.