STRUCTURE ON THE REPTILIAN TARSUS. 



155 



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

 difficulty. The fibulare and intermedium are well ossified, but 

 in the earlier forms the tibiale 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 tarsal. In Platecarpus (text-fig. 27) there are four 

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

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

 metatarsal. 



Text-figure 26. 



Text-figure 27. 



Text tig. 26. — Right tarsus and metatarsus of Mosasaurus lemonnieri Dollo. 

 A European Pythonomorph. After Dollo. 



Text-fig. 27. — Right tarsus and metatarsus of Platecarpus abruptvs Marsh. 

 An American Pythonomorph. After Williston. 



In the reptilian and amphibian tarsus 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 apparently the 

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

 Permian forms is it still present, and in no Triassic or later reptile 

 is there any trace of it. 



