Turtles and tortoises
All the members of this primitive group, which has hardly changed in 200 million years, possess a shell of bony plates covered by horny scales. This forms an arched upper shell (the carapace) and an under-shell (the plastron). Their ribs, immobile and fused to the shell, cannot be used in breathing; the necessary pumping action is provided by the abdominal muscles. There are two sub-orders.

Hidden-necked turtles
The turtles in this group withdraw their heads into the shell by flexing the neck vertically. There are seven families.

Family Chelydridae Snapper, mud and musk turtles: 23 species These turtles live on the bottom of ponds and rivers in warmer regions of the New World.
Family Dermatemydidae There is one species, the Central American river turtle, Dermatemys mawi. Little is known about this turtle, which lives in coastal rivers of Mexico and Guatamala.
Family Testudinidae Tortoises and terrapins: 115 species This family contains the most familiar land tortoises and freshwater turtles. Land tortoises are found in all the warmer parts of the world exept Australia, and in all habitats from deserts to tropical forests and oceanic islands. Most species have blunt, heavily scaled feet and high-domes carapaces.
Family Dermochelyidae There is one species, the leathery turtle or luth, Dermochelys coriacea. This 1200 lb. marine turtle lives in warm seas, but is rare. It feeds on jellyfish and tunicates, and breeds in the tropics. It has a smooth-backed appearance because its shell, unlike that of most turtles, consists of a mosaic of small bony plates embedded in skin. The shell is not joined to the ribs or the vertebrae.
Family Cheloniidae Marine turtles: 5 species These turtles live in the ocean and come ashore only to breed. The powerful front limbs are modified to form flippers, and the streamlined body narrows towards the back. The head cannot be withdrawn into the shell.
Family Trionychidae Soft-shelled turtles: 22 species These highly aquatic animals from the fresh waters of North America, Asia and Africa have no horny scales. They have partly webbed hind feet.
Family Carettochelidae There is one species the New Guinea pitted-shelled turtle, Carettochelys insculpta which is very rare and lives in rivers. It has a complete bony shell, but no covering of horny scales. It has paddle-shaped limbs.

Side-necked turtles
These turtles withdraw their heads into the shell by bending the neck sideways. There are two families.

Family Chelidae Snake-necked turtles: 31 species This group is found in South America, Australia and New Guinea.
Family Pelomedusidae Pelomedusid turtles: 14 species These turtles, from South America, Africa and Madagascar, bend their necks sideways like the snake-necks, but their necks are hidden by skin when they are withdrawn.

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