THE LIMBS 179 



trale 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 speciahzed perching Rhipto- 

 glossa (Fig. 143 a) 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 meta- 

 carpal 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 interme- 

 dium 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 a) 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 en- 

 larged. The two centralia are often present, often fused into the 

 large single bone. The fused centralia in such early forms as Idio- 

 chelys, from the Jurassic, reached almost to the radius, and the 

 radiale was doubtless cartilaginous. The fifth carpale may be ab- 

 sent, 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 coossified. 



The changes of the wrist and hand in adaptation to aquatic life are 

 more profound than those of terrestrial reptiles. The earliest ob- 

 served 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 Limnos- 



