42 



WATER REPTILES OF THE PAST AND PRESENT 



rarely sesamoid bones of any kind. The fibula, at the little-toe side 

 of the leg, is usually more slender than the tibia, though it may be 

 larger in swimming reptiles and even in some running forms. It 

 disappeared in some of the later pterodactyls. Its upper articula- 

 tion has a more gliding and somewhat rotary motion on the 



outer condyle of the femur, 

 turning the foot outward 

 in extension of the leg. 

 The tarsus of reptiles 

 differs from that of mam- 

 mals, in that the chief 

 movements of extension 

 and flexion of the foot 

 upon the leg occur within 

 the tarsus rather than 

 between the tarsus and 

 leg bones. Primitively the 

 tarsus of reptiles consisted 

 of nine bones, two in the 

 first row, two in the second, and five in 

 the third, but in all modern reptiles the 

 bones of the middle row and the fifth one 

 in the third row have disappeared; in some 

 lizards and turtles the two of the first row 

 are fused. The two bones of the proximal 

 row correspond quite to the astragalus and 

 calcaneum, the astragalus articulating with 

 both tibia and fibula proximally, the calcaneum with the latter 

 only. The eldest known tarsus of any vertebrated animal, one 

 from the Coal Measures of Ohio, has this structure, while in all 

 the early amphibians there were three bones, the tibiale, inter- 

 medium, and fibulare. Some of the later swimming reptiles, like 

 the ichthyosaurs and plesiosaurs, have apparently this amphibian 

 structure, with three bones that are usually called tibiale, inter- 

 medium, and fibulare, but it is very doubtful indeed whether 

 they are homologically the same. In the middle row two centralia 

 are known in one or two very ancient reptiles, but for the most 



Fig. 23. — Right hind 

 foot of Ophiacodon: a, 

 astragalus; c, calcaneum; 

 ci, C2, centralia; 1, 2, 3, 

 4, 5, tarsalia. 



