The three-branched shoulder, the nearly cylindrical shoulder-blade, the acrornial portion nearly equal In volume to the rest of the shoulder blade, are characteristic of the Tortoises. There is nothing parallel to this conformation In the other animals, because there is no other shoulder situated within the thorax. The varied forma of those parts afford, Cuvier observes, very good characters for the sub genera; and be details the modifications characteristic of the 'Marino Tortoises, tho Land-Tortoises, the Freah-Water Tortoises, CAdys, and Triony.r.
The humerus of the Tortoises is required to turn singularly upon its axis, in order to place the fore foot iu the position required by the bony cuirass, which only leaves a narrow passage for it. The result is that its internal tuberosity is become posterior and anterior, and that the external tuberosity is become internal and also posterior. The head of the bone goes out of the axis more than in any other animal, and that towards the posterior face which in the ordinary position is the superior one. It presents the segment of a sphere, and is very convex. The two tuberosities are very large, very pro jecting, and leave between a concavity, as there is one backwards, between the condylea of the humerus in the greater part of the mammals. The internal tuberosity—become, as has been pointed out, posterior—is the largest. It has the form of a long obtuse crest, analogous to the deltoidean, and which receives the same muscles. The other tuberosity forms a crest also, but much shorter. Both are near the head. The body of the bone is bent; and its concavity, which in man would be anterior, is ordinarily found inferior. The opposed surface is convex. Above it is a small hollow opposite the end of the fosse, which is between the two tuberosities. The lower part of the bone is widened and a little flattened from before back wards. On the external border is a furrow, not much developed in the Land-Tortoises; deeper in the Emyies, the Chelydes, and the Trionyces; and which in the Marine Tortoises nearly separates the lower head of the bone into two unequal parts. This furrow, Cuvier observes, is perhaps the best character for distinguishing the lower part of the humerus from that of the femur, which is without it, but which in every other point offers only very slight differences. Its lower head, transversely oblong and of uniform convexity, receives the bones of the fore-arm, hut without offering two distinct facets.
The Trionyees do not differ from the Land-Tortoises, excepting in having the tuberosities more apart. Other differences are manifested in Emys and Chtlys, for which we refer to Cuvier's work, but the humerus of the Marine Tortoises cannot be passed by without par ticular notice, for it differs from that of all the other Testudinota in being not bent longitudinally, but nearly straight; in having its great tuberosity (the analogue of the small or internal tuberosity in man) longer, overreaching the head, and resembling an olecranon ; and, lastly, in having the other tuberosity shorter, and representing a chevron-shaped crest.
There are always two bones in the fore-arm, but they have little motion one on the other. They are placed, when the animal pro gresses, so that the ulna forms the external and the radius the internal border of the arm.
The radius has a semicircular, slightly concave, upper head, a somewhat slender body, and the lower head compressed and cut, as it were, obliquely, so that it is shorter on the ulnar side.
The ulna is compressed. Its upper head is triangular and cut obliquely, so that its external border is longer upwards than the radial border without having a true olecranon. This border is tren chant. The lower one is cut square. Differences occur, as in Trionyx and the Citc/ones, or Marine Tortoises.
The pelvis is always composed of three distinct bones, contributing, as in the Mammalia, to the composition of the cotyloid fossa, namely, an elongated os ilium, which attaches itself by ligaments to the transversal processes of the sacral vertebra and the neighbouring part of the eighth pair of the dilated ribs ; a pubis and an ischium, which are directed, widening as they proceed towards the plastron, and are each united to its similar piece. At the point of union fur the formation of the cotyloid cavity, each bone has three faces; one for each of the two others and one for the cavity. On the rest of the length the os ilii is oblong, the ischium proceeds, wideliing as it goes. directly towards the symphyais, and the pubis, after first directing itself forward, makes a curve towards the symphysis, and widens also to reach it. Various differences occur in this part of the skeleton in the Land and Marino Tortoises, in Chelya, and in Trionyx.