The skin which covers the body of these animals sometimes preserves its softness, being altogether devoid of scales; but in nearly all the species it is covered with horny scales of great consistency. Upon the plastron and carapace these scales form large plates, the arrangements and appearances of which vary in different species, some of them being often remarkably beautiful. The material which bears the name of Tortoise-shell forms an important article of commerce. * * * * * Aristotle mentions three groups of Tortoises; namely, +chelôêchersaia+, or Land Tortoises; +thalattia+, or Sea Tortoises; and +Eôhus+, or Fresh-water Tortoises. Cuvier divides them into five sub-genera:--1, Land Tortoises, _Testudo_ (Brogniart); 2, Fresh-water Tortoises, _Emydes_ (Brogniart); 3, Marine Tortoises, _Chelque_ (Cuvier); 4, Chelydes, _Testudo fimbriata_; 5, Soft Tortoises, _Trionyx_ (Geoffrey)--in which he is followed by Dr. Gray in the British Museum Catalogue, who makes them the third order of Reptiles in his arrangement as follows:-- I. TESTUDINIDÆ. Testudo, Chersina, Kinixys, Pyxis. II. EMYDIDÆ. Geoemyda, Emys, Cyclemys, Malaclemys, Cistudo, Kinosternon, Chelydra, Platysternum. III. CHELYDIDÆ. Sternotherus, Pelomedusa, Hydraspis, Chelymys, Phrynops, Chelodina, Hydromedusa, Chelys, Peltocephalus, Padocnemis. IV. TRIONYCIDÆ. Trionyx, Emyda. V. CHELONIADÆ. Sphargis, Chelonia, Caretta, Casuana. In the valuable "Erpétologie" of Messrs. Duméril and Bibron, the Chelonians are divided into--1, Land Tortoises, _Chersites_; 2, Marsh Tortoises, _Elodites_; 3, River Tortoises, _Potamites_; 4, Sea Tortoises, or Turtles, _Thalassites_. This arrangement being the most simple, is adopted as best adapted to our purpose.] LAND TORTOISES. Terrestrial Tortoises are distinguished by their short, oval, and convex bodies, covered by carapace and plastron; four feet, and the absence of teeth; short, stumpy, unshapely legs; nearly equal toes, armed with claws, united by a thick skin, so as to form a clumsy foot, the periphery of which forms a sort of hoof, which seems adapted for the land. In this group the carapace is very convex, its height sometimes exceeding its breadth; it forms a solid, generally an immovable, arch, under which the animal can completely conceal its feet and tail. This _buckler_ is covered with large horny plates or scales. Land Tortoises have been known from the earliest times, representations of them being found on numerous monuments of antiquity, the product of ancient art. Moreover, ancient writers tell us that the carapace of the Tortoise contributed its substance to the formation of the first lyre; it was consequently sacred to Mercury as the deity of music and inventor of that instrument.