Bones of the Trunk : Dorsal Buckler, or Carapace.—The wide dif ferences prevalent in the modification and arrangement in the bones of the head in this order lead one to expect, as the great French zoolo gist observes, proportional differences in tho rest of the skeleton. Tho cranial differences are, as he remarks, greater perhaps than obtain among the whole of the mammals, and most certainly are more extensive than can be foutid in the whole class of birds.
The general distinguishing e.haracte,r of the Tortoises, that which separates them from all the rcricbraia, is the external position of the bones of the thorax, enveloping with a cuirass or double buckler tho muscular portion of the frame, and serving also as a protection for the shoulder-bones and the pelvis.
The dorsal buckler is principally formed of eight pairs of ribs, united towards the middle by a longitudinal succession of angular plates, which adhere to the annular parts of so many vertebrae, or even form a part of them ; hut it is remarkable that these annular portions alternate with the body of the vertebrae, and do not correspond directly with them.
Tho ribs are inlaid by means of sutures into those plates ; they are also united with each other, on the whole or a part of their length, according to the species, and even in each species according to the ages of the individuals. There are eight anterior vertebra, which do not enter into this conjunction. The first seven (the ordinary cer vical) are free in their movements. The which may be regarded as the first dorsal, is placed obliquely between the last cervical and the first of the fixed vertebra) of tho dorsal buckler, which shortens it anteriorly; behind, its spinous apophysis is elon gated, and enlarges a little to attach itself by synchondroeis to a tubercle of the first of tho plates of the intermediate series of the phistron.
The first of these fixed vertebra), which is the second dorsal, is still rather short, and carries also its proper annillur part, the spinous apophysis of which, shorter than the preceding, attaches itself to the second plate by a cartilage. This second plate, narrower than the first, forma but ono bone with an annular part which and of which the anterior portion is articulated by two small apophyses with the articular apophyses of the second dorsal. This, properly speaking, is the annular portion• of the third dorsal vertebra; but the body of this third vertebra is only articulated by its anterior moiety with the posterior moiety of this third annular part, and by its posterior moiety it is articulated to the anterior moiety of the fourth annular portion ; and this alternative continues, so that the body of the fourth vertebra responds to the annular portions of the third and the fourth, the body of the fifth to the annular portions of the fourth and fifth, and so on to the tenth.
But it is necessary to distinguish in the ribs the plate included in the buckler, and a small branch which proceeds from its lower sur face, and which represents what is termed the head of the bone in the ordinary ribs. This bead is always articulated between two bodies of vertebra. The first of all these ribs has only this small branch, with out having any plate belonging to it in the buckler, excepting only in some of the Emydes, where may be seen, between the first and second longitudinal plate, and the first or second widened rib, a small piece which can only represent the enlarged portion of this first rib, but which does not belong to its head_ It is articulated between the eighth vertebra or first dorsal, and the first fixed vertebra, and by its other extremity applies itself to the internal surface of the second rib. This last ha a plate which incorporates itself by its anterior border with the first of the longitudinal series, by its spinal border with the second piece of that series or the annular portion of the third vertebra, and by its head between the body of the second vertebra and that of the third. The succeeding ribs observe the same law, are articulated by means of their head between the body of one vertebra and that of the succeeding vertebra, and incorporate themselves by means of their dilated part with the plate which represents the annular portion of the second of these two vertebra): and this, Cuvier observes, is a return to the general Law ; for in man and in the quadrupeds the ribs are articulated by their head between two vertebr,e, and by means of their tuberosity, with the transverse apophysis of the second of the two. The dilated portions of the ribs of the tortoise, in the part where they are incorporated with the plates of the longitudinal series, represent, then, the tuberosities of the ribs of mammals. The ninth plate of the longitudinal series, which belongs to the tenth dorsal, is the last with which a pair of the dilated ribs is incorporated ; and this last is the ninth in all, or the eighth of those which enter into the composition of the dorsal buckler. It is directed from its posterior border backwards, and embraces again the succeeding plates, with the external edges of which it becomes incorporated : but these three plates do not, any more than the first, serve to complete the vertebral canal.