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an order of turtles, comprising a single family, the Cheloniidae (sea turtles). These large animals are thoroughly aquatic, leaving the water only to lay their eggs on the shore. The upper shell, or carapace, is low and streamlined. The head, which is on a short neck, does not completely retract under the carapace. The limbs have been modified into flippers; the anterior flippers are oar-shaped and longer than the posterior ones. The plastron and the carapace are not attached.
Sea turtles are distributed in tropical and subtropical seas; they rarely enter temperate latitudes. There are four genera, each having one species and two subspecies (Atlantic and Pacific subspecies). The largest species, the green turtle (Chelonia mydas), weighs up to 400 kg and has a carapace measuring up to 1.4 m long. Because it is commercially valuable for its tasty meat and eggs, it has been greatly reduced in number. The loggerhead (Caretta carettd) has a massive head, and its carapace is up to 1 m long. The species Lepidochelys olivacea is characterized by a rounded carapace, measuring up to 80 cm in length. The carapace of the hawksbill turtle (Eretmochelis imbricata) is up to 85 cm long.
Sea turtles swim rapidly and well and are able to dive to considerable depths. They travel hundreds of kilometers in search of food and suitable places for mating. The female deposits anywhere from several dozen to 200 (or more) spherical eggs, which are covered with a leathery membrane.
Sometimes the family Dermochelydidae—with its single species, the leatherback (Dermochelys coriaced)—is included in the order Chelonioidea.