Tortoises and Turtles constitute the Reptilian order Chelonia, well marked off from all the other orders by the possession of a dorsal shield, or carapace, and a ventral shield, or plastron, united at the sides, and forming a kind of case, within which the head and neck, limbs (of which there are always two pairs fitted for walking or swimming), and tail can be completely or partially retracted. The carapace is usually covered with hard horny plates called tortoise-shell. The jaws are toothless, and covered with horny sheaths, like the bill of a bird. There are two sub-orders:- 1. Athecata, in which the carapace is flexible, with but one living form, the Leathery Turtle (Sphargis coriacea), which has a wide range in temperate and tropical seas, and is said to live enitrely on marine vegetation. It sometimes attains a length of six feet, and the carapace is marked by seven grooves running backwards from the head. 2. Testudinata, in which the carapace is hard and rigid. These are generally divided into four families. (1) Cheloniidae, exclusively marine, with the shields of the carapace partially ossified and the feet large and fin-like. They are natives of intertropical seas. The two most important forms belong to this family - the Green Turtle (Chelone viridis) from the Atlantic, the most highly prized of those used for food, and the Hawk's-bill Turtle (Caretta imbricata) from the Indian and Pacific Oceans, from which is obtained the tortoise-shell of commerce. (2) Testudinidae, the Land Chelonians. One of the best-known forms is the Common Greek Tortoise, often hawked about the streets of London and other large towns. To this family belong the gigantic tortoises of the Galapagos. These attain a length of about three feet. They are rapidly becoming extinct, owing to the fact that their flesh is excellent eating, and that a valuable oil is prepared from their fat. (3) Chelydidae, containing forms frequenting marshes and fresh water. T this family belong some European forms and the terrapins of America. (4) Trionychidae, fresh-water turtles carnivorous in habit, having the carapace covered with soft skin. The webbed digits end in sharp claws. The name "tortoise" appears to have been originally applied to terrestrial, and "turtle" to marine forms; but, though apparently so different, the origin of both words is the same. The meaning is "the animal with twisted feet." The commercial product has always been known as tortoise-shell," though obtained from a marine chelonian. It is used for combs, jewellery, and inlaying, but much less extensively than formerly, perhaps because it is so easily imitated.