STRUCTURE ON THE REPTILIAN TARSUS. 



155 



The Plesiosanrian tarsus (text-figs. 24, 25) presents little 

 difficulty. The fibulai-e and intermedium are well ossified, but 

 in the earlier forms the tibia le is often partly or wholly cartila- 

 ginous. In later types the tibiale is well developed. There are 

 only three distals, which I believe to be first, third, and fourth. 

 The fifth metatarsal is shortened up as in Chelonians. 



The Pythonomorphs show a most interesting type of tarsus. 

 In Mosasaurus (text-fig. 26) there are only three tarsal elements, 

 which are manifestly the fibulare, the intermedium, and the 

 fourth distal tiirsal. In Platecarjms (text-fig. 27). there are four 

 tarsal elements— the fibulare, intermedium, and the third and 

 fourth distal tarsals. In botli types there is a specialized fifth 

 metatarsal. 



Text-fia-ure 26 



Text fig. 26. — Rig-lit tarsus and metatarsus of 3Iosasaurus lemonnieri Dollo. 

 A European Pythonomorpli. After Dollo. 



Text-fig. 27.— Riglit tarsus and metatarsus of Flatecarpus abrnptvs Marsli. 

 An American Pythonomorpli. After Williston. 



In the reptilian and amphibian tai'sus the most remarkable 

 features are the almost constant presence of the fibulare and the 

 intermedium, and the great variability of the tibiale. Rarely is 

 the tibiale a large element : very frequently it remains entirely 

 or partly cartilaginous. In many types it is completely absent. 

 In the mammal-like reptiles and in the primitive Diapsidans it is 

 wedged in between the intermedium and the first and second 

 tarsalia. 



The central elements, of which there are four in some 

 amphibians, are early greatly reduced and lost. The only one 

 which for a time remains in early reptiles is apparent!}^ the 

 homologue of the proximal centrale in Trematops. In only a few 

 Permian forms is it still present, and in noTriassic or later reptile 

 is there any trace of it. 



