one anterior, the other posterior, hinged on an in

termediate fixed piece, as in the annexed outline; so that the animal can shut itself completely in: a, the anterior mobile portion; b, the posterior; c, the middle fixed portion. In the genus Cistudo, there is only one hinge dividing the plastron into two moveable parts: in the genus Sternotherus, the anterior part only of the plastron is moveable.

In the Matamata, (Chelys matamata,) the plastron is narrow, elongated, and firmly consolidated to the costal or rib plates of the carapace. In the Emysaurus serpentinus, the plastron does not form a complete covering to the abdomen: it is narrow and terminates anteriorly in a point, which is enveloped in the skin, but its middle portion extends to meet on each side the edge of the carapace. In the river Tortoises, as the Trionyx, (Gymnopus, Bibron,) the osseous part of the plastron is imperfectly developed, and is bordered all round by a tough leathery skin, which unites it to a similar skin bordering the imperfect carapace. In marine Tortoises, or Turtles, the plastron is united to the edges of the carapace by intervening cartilage, and not by suture. We might here proceed to the enumeration of many other modifications of form which the plastron exhibits, but we have said enough to show the general extent to which it varies. In like manner the carapace exhibits numerous variations of form and degrees of developement, from the dense and solid structure of the land Tortoises, to the restricted expansion of its component parts in the Matamata, or in the common Turtle; from a vaulted figure to a flat,

or nearly flat, expanse, and through different grades of solidity. Suffice it to say, that in the land Tortoises it is the strongest, the most arched, and the most completely developed; and that in these it constitutes a solid protection, which no ordinary pressure can break. We have said that both the carapace and the plastron are sheathed externally with horny plates; these are differently arranged; sometimes they overlay each other, like great scales, as in the Hawk's-bill Turtle, (Chelonia imbricata.) The plates of this animal are valuable in a commercial point of view, constituting what is commonly called tortoise-shell. Generally, however, the plates are merely laid on the bone, and touch each other only at their edges; they vary in form, and in the direction of the raised lines, and intervening furrows with which their surface is ornamented. It is almost unnecessary to say, that these horny epidermic plates, (upon the skin, em, epi, upon; depua, derma, the skin,) are the result of a process of secretion regularly carried on; and when one is removed during the life of the animal, it is gradually reproduced. To a certain extent these plates correspond to the bones composing the carapace and plastron, but not entirely; they do not terminate along the line of the different sutures, but pass over them; and indeed they seem by extending beyond the line where the bones unite, as if designed to add to their consolidation into. one. The sketches on the next page will give a good idea of their ordinary arrangement: the numbers refer to the names by which naturalists distinguish them.

A. The carapace of the European Marsh Tortoise, or Emys.-B. The carapace of the Loggerhead Turtle, (Chelonia caouana.)-c. The plastron of the European Marsh Tortoise.-D. The plastron of the Loggerhead Turtle, (Chelonia caouana,) a marine species.

Figs. A and B.-1, 2, 3, 4, 5, the vertebral plates or scales; 6, 7, 8, 9, the costal plates; 6, A, an additional costal plate, met with in some marine turtles; 10, the nuchal plate; 11, the caudal plate, sometimes single

sometimes double; 12, the anterior marginal, or marginal neck plates; 13, 14, the marginal arm plates; 15, 16,

17, 18, 19, the marginal side plates; 20, 21, 22, the marginal thigh plates.

Figs. c and D.-1, The gular or throat plate, sometimes

single, sometimes double; 1 a, the intergular plate. This, when present, is situated sometimes before, sometimes behind, the gular plates; 2, the humeral plates; 3, the pectoral plates; 4, the abdominal plates; 5, the femoral plates; 6, the subcaudal plates; 7, the axillary plates; 8, the inguinal plates; 9, 10, 11, 12, 13, the sternolateral plates, which are present only in marine Turtles

Having thus explained the nature of the carapace and plastron, and of the scales or plates which form an external covering to the consolidated spine, ribs, and sternum, of which these parts are actually composed, we may proceed to notice the manner in which respiration is effected; as it is, in a great measure, by the action of these parts, namely, the ribs and sternum, in mammalia, that the lungs become filled with air. Mammalia, for example, breathe by expanding the cavity of the chest occupied by the lungs, into which, accordingly, the air enters, through the trachea, or windpipe, to fill up the vacuum occasioned by the dilatation of the cavity. Now, this action, as is very manifest, supposes a certain degree of mobility in the walls of the chest, or, in other words, of the ribs and sternum which encircle it, independently of the action of the great muscle of respiration, or diaphragm, which parts the chest from the abdominal cavity. In Tortoises, the walls of the chest are, as we have shown, immoveable, they cannot be expanded, the bones are all locked into one solid mass; and there is no muscular diaphragm, parting the cavity occupied by the heart and lungs, from that containing the rest of the viscera. There must, therefore, be some peculiar mechanism, by which the lungs become filled. The fact is, that the air is forced by the action of the tongue and mouth, through the trachea into the lungs, by an act resembling deglutition, or rather, in the manner in which the ball, or hollow butt, of an air-gun is charged by repeated strokes of the piston.

The jaws being firmly closed, the cavity of the mouth

is enlarged by the drawing down of the root of the tongue and of the hyoid bone* which supports it, and the air at the same time rushes into the mouth through the nostrils. The free part of the tongue is now applied to the posterior orifices of the nostrils, so as to stop them, as though by a valve; the gullet, also, is now closed, the root of the tongue is elevated, the broad muscles of the throat contract, and the air is forced down the windpipe into the lungs; which become filled by a repetition of the process. The breathing out, or expiration of the air, is effected by the simple pressure of the abdominal and other muscles within the plastron and carapace. It is in this manner that the Amphibia, which have no ribs, also respire, as the frog, and toad, etc. The lungs of Tortoises are of ample volume; they are two in number, and are placed immediately beneath the carapace, and above the rest of the viscera. To the aquatic species, in particular, the dorsal situation of the lungs is of great advantage; these organs act as a float, and enable the animals to retain their due position among the roughest waves: for it must be very evident, that the part containing voluminous lungs filled with air, will be always uppermost in the

water.

The jaws of Tortoises are not armed with teeth, but are cased in horny coverings, resembling the sharp, hooked beak of a parrot, with which they crop and mince the vegetable aliment on which they feed. In some species, as the Trionyx, around the outside of this beak are thick fleshy lips; the food consists of small living animals, as amphibia, young birds, reptiles, etc. In the Matamata the jaws which open very wide, instead of being armed with a strong beak, are protected with a sheath of horn so delicate, that most naturalists have overlooked it, and Cuvier states that the mouth of this Tortoise resembles that of the Rana pipa. We select

Os hyoides, so called from resembling the Greek letter v; it supports the roof of the tongue.