["The body," as described by Dr. Gray, "is covered with square imbedded plates, generally forming a dorsal and ventral shield united by their margins, leaving only the head, neck, limbs, and tail free, and (in some species, as the Box-Tortoises, _Cinasternon_, shut up by movable closely-fitting doors) only covered with a scaly skin; the upper shield formed of the ribs united together and adherent to the dorsal vertebræ by a toothed suture, and surrounded by a series of bones forming the edge of the shields; the lower shield, or sternum, formed of four pairs and a central anterior bone; the jaws toothless, covered with a horny bill, rarely hid by fleshy lips; eyelids distinct; drum of the ear visible; legs short and thick; tail conical." "The natural dwelling chamber of the Chelonia consists chiefly," says Professor Owen, "and in the marine species (_Chelone_) and Mud Turtles (_Trionyx_) solely, of the floor and the roof; side-walls of variable extent are added to the fresh-water species (_Emydians_) and Land Tortoises (_Testudinians_). The whole consists of 'osseous plates,' with superincumbent horny plates, or 'scutes,' except in the Soft or Mud Turtles (_Trionyx_ and _Sphargis_), in both of which these are wanting."--("Circle of the Sciences.") These animals, to which a portable stronghold is thus given in compensation for inferior powers of locomotion and defence, are recognisable at a glance from the singular armour with which Nature has provided them. A double shield envelopes all parts of their bodies, only permitting the head, neck, legs, and tail to pass through it: moreover, all these organs can be hidden within this double cuirass by means of a retractile power possessed by the animal. This double armour consists of a _carapace_, or back-piece, and _plastron_, or breast-plate, composed of a series of small bones or plates closely united together; the first resulting from the union of the sides and dorsal vertebræ, the plastron, or lower buckler, being only a highly-developed sternum. These organs are merely portions of the skeleton, which, in place of being lodged in the depths of the soft parts, has become the superficies, which is only covered by a thin, dry skin. This numerous and highly-interesting order of Reptiles, called Chelonia, from +chelônê+, a Tortoise, are also called _Testudinata_, from _testudo_, the Latin name for a Tortoise, from the double shield in which the bodies of all, whether terrestrial, fresh-water, or marine Tortoises, are enclosed.