i8o THE OSTEOLOGY OF THE REPTILES 



cells (Fig. 133) and Diadectes, probably marsh animals. In Clidastes 

 (Fig. 146) of the Mosasauria, essentially a surface-swimming lizard, 

 the four proximal bones of the wrist are ossified, but the centralia, 

 first and fifth carpalia were not. In Platecarpus (Fig. 147) a more ad- 



FiG. I45. Chelonia, Pleurodira: Tw/Wo, A, front leg, dorsal side; B, the same, radial side; 

 C, hind foot (tarsus, etc.) dorsal side. 



vanced aquatic type, the ulnare, pisiform, and second carpale have 

 also disappeared, leaving only the radiale, intermedium, third and 

 fourth carpalia. In Tylosaurus (Fig. 148) the most highly specialized 

 of all mosasaurs, there are but one or two bony nodules left, one of 

 which is certainly the fourth carpale. All the others disappeared as 

 bones but remained as cartilage, since space is left for them in many 

 specimens as they have been found in the rocks. 



