Turtles, tortoises, and terrapins
Turtles are the only living members of the subclass Anapsida, which is characterized by a primitive skull with a solid cranium and no temporal openings (anapsid). The most exceptional feature of the turtle is its shell; this extremely conservative character has remained little changed for about 200 million years. These turtles constitute the order Testudines, the most primitive order of living reptiles, which apparently is close to the ancestral reptilian lineage (as indicated by its unspecialized skull). Living turtles are found on all continents except Antarctica. The marine species occur predominantly in tropical waters of the Atlantic, Pacific, and Indian oceans, but several also range far poleward in these waters. In maximum size, adult turtles range from less than 10 cm (Homopus signatus) to nearly 300 cm and over 900 kg (Dermochelys coriacea).
All living shelled reptiles are turtles, but the terms tortoise and terrapin have also been applied, and these have different meanings in various parts of the world. Tortoise is best applied to terrestrial turtles. Terrapin is usually applied to edible, more or less aquatic, hard-shelled turtles.