THE LIMBS 



193 



In the web-footed Mosasauria the tarsus, like the carpus (Figs. 

 146-148), progressively became more cartilaginous. In Platecarpus 

 (Fig. 158 a) and Clidastes (Fig. 158 b) the astragalus, calcaneum, and 

 fourth tarsale alone remain, with the divaricated fifth metatarsal, as 

 in land lizards. In Tylosaurus, the most speciahzed of mosasaurs, 

 but one, or at most two, small bones remain. Other tarsal bones 

 remained unossified, though represented by cartilage in the adult. 



Fig. 158. Limbs of aquatic reptiles: A, Platecarpus (Mosasauria), right hind leg. About 

 one sixth natural size. B, Clidastes (Mosasauria), right hind leg and tarsus. One third 

 natural size. C, Ophthalmosaurus (Ichthyosauria), left front paddle. One eighth natural 

 size. D, Ichthyosaurus platydactylus (Ichthyosauria), left front paddle. One sixth 

 natural size. 



Not more than six bones of the plesiosaurs can be called tarsals, 

 and their homologies are doubtful (Fig. 159 b, c). They have the 

 same shapes and relations as the carpal bones and cannot be dis- 

 tinguished from them except by their smaller size. The three in the 

 first row are usually called the tibiale, intermedium, and fibulare; a 

 fourth, on the postaxial side, has sometimes been called the pisiform 

 in both front and hind limbs, but as there never was in any terrestrial 

 reptiles a pisiform in the tarsus, that name is of course incorrect. 



