148 



DR. It. BROOM ON THE 



difficulties. The two elements in the centre of the tarsus, 

 however, may readily give rise to difference of opinion. By 

 Williston, Case, and others who have written on the type they 

 are regarded as first and second centralia. The inner of the two 

 is the element which becomes the mammalian navicular. The 

 outer one is an element which becomes early lost, and is only 

 known in a very few Permian forms. 



If we identify, as I think we must do, the astragalus with 

 the intermedium, we must either regard the tibiale as lost, or find 

 it is the inner of the two supposed central elements. This inner 

 element supports the first tarsal, and though it appears to have 

 slipped away from the tibial articulation, it is still not far 

 removed from the tibia. If we are right in identifying the inner 

 proximal elements in Scincosaurus and Seymouria as the tibiale, 



Text-fieure 8. 



Text-figure 9. 



Text-fig. 8. — Right tarsus and metatarsus of Ophiacodon mirus Marsh *. 

 After Williston. Slightly modified. 



Text-fig. 9. — Right tarsus and metatarsus of Casea broilii Williston f. 

 After Williston. 



* An early Theromorph. 



t An aberrant Theromorph. 



then there is good reason to believe that the navicular of the 

 Pelycosaurs, the Therapsids, and the Mammals is also the 

 tibiale which by the lengthening and narrowing of the tarsus 

 has become slightly altered in position. In the'Cotylosaurs the 

 distal tarsals are nearly twice as wide as the fibulare and the 

 intermedium. In the more active Pelycosaurs the tarsus has 

 become so narrowed that the distal tarsals together measure often 

 less and rarely much more than the width of the two large 

 proximal elements. If the tibiale is to be retained at all it can 

 only be by becoming wedged in between the intermedium and the 

 first and second tarsalia. This, I believe, is what has happened ; 



