celis (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 advanced 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.
Fig. 145. Chelonia, Pleurodira: Testudo, A, front leg, dorsal side; B, the same, radial side; C, hind foot (tarsus, etc.) dorsal side.
