Chelonia mydas in Kona, Hawaii
The largest living chelonian is the leatherback sea turtle (Dermochelys coriacea), which reaches a shell length of 200 cm (6.6 ft) and can reach a weight of over 900 kg (2,000 lb). Freshwater turtles are generally smaller, but with the largest species, the Asian softshell turtle Pelochelys cantorii, a few individuals have been reported up to 200 cm (6.6 ft). This dwarfs even the better-known alligator snapping turtle, the largest chelonian in North America, which attains a shell length of up to 80 cm (2.6 ft) and weighs as much as 113.4 kg (250 lb). Giant tortoises of the genera Geochelone, Meiolania, and others were relatively widely distributed around the world into prehistoric times, and are known to have existed in North and South America, Australia, and Africa. They became extinct at the same time as the appearance of man, and it is assumed humans hunted them for food. The only surviving giant tortoises are on the Seychelles and Galápagos Islands, and can grow to over 130 cm (51 in) in length, and weigh about 300 kg (660 lb).
The largest ever chelonian was Archelon ischyros, a Late Cretaceous sea turtle known to have been up to 4.6 m (15 ft) long.
The smallest turtle is the speckled padloper tortoise of South Africa. It measures no more than 8 cm (3.1 in) in length and weighs about 140 g (4.9 oz). Two other species of small turtles are the American mud turtles and musk turtles that live in an area that ranges from Canada to South America. The shell length of many species in this group is less than 13 cm (5.1 in) in length.
A red-eared slider turtle with eyes closer to the end of the head, keeping only the nostrils and the eyes above the water surface
African spurred tortoise in the zoo of Sharm el-Sheikh
African spurred tortoise at a zoo in the Czech Republic
Turtles are divided into two groups, according to how they evolved a solution to the problem of withdrawing their necks into their shells (something the ancestral Proganochelys could not do): the Cryptodira, which can draw their necks in while contracting it under their spine; and the Pleurodira, which contract their necks to the side.
Most turtles that spend most of their lives on land have their eyes looking down at objects in front of them. Some aquatic turtles, such as snapping turtles and soft-shelled turtles, have eyes closer to the top of the head. These species of turtles can hide from predators in shallow water, where they lie entirely submerged except for their eyes and nostrils. Near their eyes, sea turtles possess glands that produce salty tears that rid their body of excess salt taken in from the water they drink.
Turtles are thought to have exceptional night vision due to the unusually large number of rod cells in their retinas. Turtles have color vision with a wealth of cone subtypes with sensitivities ranging from the near ultraviolet (UV A) to red. Some land turtles have very poor pursuit movement abilities, which are normally reserved for predators that hunt quick-moving prey, but carnivorous turtles are able to move their heads quickly to snap.
Turtles have rigid beaks, and use their jaws to cut and chew food. Instead of having teeth, the upper and lower jaws of the turtle are covered by horny ridges. Carnivorous turtles usually have knife-sharp ridges for slicing through their prey. Herbivorous turtles have serrated-edged ridges that help them cut through tough plants. They use their tongues to swallow food, but unlike most reptiles, they cannot stick out their tongues to catch food.
Main article: Turtle shell
The upper shell of the turtle is called the carapace. The lower shell that encases the belly is called the plastron. The carapace and plastron are joined together on the turtle's sides by bony structures called bridges. The inner layer of a turtle's shell is made up of about 60 bones that include portions of the backbone