The plastron, or ventral plate, is made up of nine pieces, of which eight are arranged in pairs; but the ninth, which is always placed between the four pieces composing the two anterior pairs, is single, and occupies the mesial line. The bones of the shoulder and hip are placed within the thorax, and articulated to the sides of the vertebral column.
Of this vertebral column in these extraordinary animals, Professor Owen remarks that the manifold modifications of the framework which render it a portable abode, appear to have been given as a compensation for inferior powers of locomotion, and the absence of offensive weapons. But with all its modifications, the same number of pieces are found in the bony skeleton as in other ordinary vertebratæ, the form and volume of many of these pieces being alone changed.
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, χελωη χερσαια, or Land Tortoises; θαλαττια, or Sea Tortoises; and Εωhυς, 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.