Page:The Osteology of the Reptiles.pdf/197

Rh is also identified in some lizards. The fourth centrale [carpale], as usual, is large; the second, third, and fifth are usually large. The first is absent, unless it be the element sometimes called the first centrale.

In the curious hands of the highly specialized perching Rhiptoglossa (Fig. 143 ) the carpus is reduced to four functional bones, the radiale, ulnare, and posteriorly placed pisiform in the first row, and a large, hemispherical, fused third and fourth carpale in the distal row, around which the metacarpals revolve. Between the first metacarpal and radiale there are in the more specialized types two minute bones, which may represent the first and second carpalia, or the second and the centrale, probably the latter.

In the marine Chelonia (Fig. 144) the carpus is broad and flat, and is least reduced, though much modified. The radiale and intermedium are more or less elongate, the ulnare is small, the centrale large. The pisiform is greatly enlarged and has lost its primitive location between the ulna and ulnare, becoming attached to the ulnare and fifth metacarpal or the latter alone. This was the structure of the marine turtles as far back as the Cretaceous in Protostega, except that the proximal bones were less elongate.

At the opposite extreme, among the terrestrial tortoises (Fig. 145 ) the radiale has disappeared until nothing is left of it but a nodule of cartilage united with the first centrale, which has usurped its place. At least, this is the explanation given by Baur, who found in Emydura the two centralia in their normal positions, though enlarged. The two centralia are often present, often fused into the large single bone. The fused centralia in such early forms as Idiochelys, from the Jurassic, reached almost to the radius, and the radiale was doubtless cartilaginous. The fifth carpale may be absent, fused with the fourth, or separate and distinct. Indeed, in some old animals the third, fourth, and fifth carpalia and the pisiform may all be coössified.

The changes of the wrist and hand in adaptation to aquatic life are more profound than those of terrestrial reptiles. The earliest observed effect of water habits is delayed ossification, not only of the mesopodial bones, but of the bones of the skeleton in general, a large amount of cartilage remaining in the joints. Partial chondrification of the wrist and ankle occurred as early as the cotylosaurian