Both the carapace and plastron are covered by a series of horny plates (rarely wanting), which are developed in the epidermis, and which are perfectly distinct from the bones which they cover. As encasing the upper surface of the carapace, these plates (which in some species constitute the "tortoise-shell" of commerce) have a general arrangement conforming with that of the bony plates beneath, though there is no numerical correspondence between the two. Thus the carapace, as we have seen, consists of (1) a median series of "neural" plates developed from the vertebrae; (2) a lateral series of " costal" plates on each side, corresponding with and largely formed by the ribs; and (3) a peripheral series of "marginal" plates (see fig. 293). Similarly, the epidermic plates (fig. 293) are arranged in (1) a median, "vertebral," or "neural " series; (2) a lateral series on each side, of "costal" scutes; and (3) a series of "marginal" scutes. The "vertebral" scutes, however, are only five in number; and each series of "costal" scutes consists only of four pieces, so that the number of epidermic plates is much smaller than that of the bony plates beneath. The "marginal" scutes, on the other hand, correspond in number with the "marginal plates " beneath them. They are, therefore, twenty-four or twenty-six in number, the anterior scute in the middle line being distinguished by the epithet of "nuchal," while the corresponding scute behind is termed "pygal."
Fig. 293. - Carapace of the Loggerhead Turtle (Chelone caouanna), viewed from above (after Owen). In this form, the ribs are separate and free towards their extremities, and the osseous portions of the carapace are indicated by the light lines, while the epidermic plates are marked out by dark lines. n n The first and last of the median series of "neural plates ;" c c The expanded ribs or "costal plates;" m m The first " marginal plate" on each side; nu Nuchal plate; py Pygal plate; v v Median series of epidermal plates, or "vertebral scutes."
The other points of importance as regards the endoskeleton are these:
Firstly, The dorsal vertebrae are immovably joined together and have no transverse processes, the heads of the ribs uniting directly with the bodies of the vertebrae.
Secondly, The scapular and pelvic arches, supporting the fore and hind limbs respectively (fig. 290, s and p), are placed within the carapace, so that the scapular arch is thus inside the ribs, instead of being outside, as it normally is. The scapular arch consists of the shoulder-blade or scapula, and two other bones, of which one corresponds with the acromion process of human anatomy, and the other to the coracoid process, or to the "coracoid bone," of the Birds. The clavicles, as is also the case with the Crocodilia, are absent; but the three anterior pieces of the plastron may represent an inter-clavicle and clavicles.
The order Chelonia is conveniently divided into three sections, according as the limbs are natatory, amphibious, or terrestrial. In the first of these, the limbs are converted into most efficient swimming-paddles, all the toes being united by a common covering of integument. In this section are the well-known Turtles (Cheloniidae), all of which swim with great ease and power, but are comparatively helpless upon the land (fig. 294). The legs are of unequal length, and the carapace is much depressed and flattened. The best-known species are the "edible" or Green Turtle (Chelone midas), the Loggerhead Turtle (Chelone caouanna), the Hawk's-bill Turtle (C. imbricata), and the Leathery Turtle (Sphargis coriacea). The Green Turtle is largely imported into this country as a delicacy, and occurs abundantly in various parts of the Atlantic and Indian Oceans. The Hawk's-bill Turtle is of even greater commercial importance, as the horny epidermic plates of the carapace constitute the "tortoise-shell" so largely used for ornamental purposes. The Leathery Turtle is remarkable in having the carapace covered with a leathery skin in place of the horny plates which are found in other species.
In the second section of the Chelonia, in which the limbs are adapted for an amphibious life, are the Mud-turtles or soft Tortoises (Trionycidae), and the Terrapins (Emydidae). In the Trionycidae the development of the carapace is imperfect, the ribs being expanded and united to one another only near their bases, and leaving apertures near their extremities. The entire carapace is covered by a smooth leathery skin, and the horny jaws are furnished with fleshy lips. All the Trionycidae inhabit fresh water and are carnivorous in their habits. Good examples are found in the Soft-shelled Turtle (Trionyx ferox), and the large and fierce Snapping Turtle (Chelydra serpentina) of the United States; but other species are found in
Fig. 294. - Hawk's-Bill Turtle (Chelone imbricata) - after Bell.
Egypt and the East Indies. The Terrapins (Emys) have a horny beak, and have the shield covered with epidermic plates. They are inhabitants of fresh water, and are most of them natives of America.
The third section of the Chelonia comprises only the Land Tortoises (Testudinidae), in which the limbs are adapted for terrestrial progression, and the feet are furnished with short nails. The carapace is strongly convex, and is covered by horny epidermic plates; the head, limbs, and tail can be completely retracted within the carapace. Though capable of swimming, the Tortoises are really terrestrial animals, and are strictly vegetable feeders. The most familiar species is the Testudo Graeca, which is indigenous in Spain, Italy, and Greece. A much larger species is the so-called Indian Tortoise (Testudo Indica), which inhabits the Seychelles and Galapagos Islands, and attains a length of over three feet.