STRUCTURE OF THE REPTILIAN TARSUS. 



145 



more reaclil}^ regard them as the fibulare, tibiale, and centrale 

 from their superficial resemblance to the calcaneum, astragalus, 

 and navicular of the mammalian tarsus. It is, however, quite 

 manifest that, whatever the tibia supports, it is not the element 

 lying on the tibial side of the fibulare. We can readily see that 

 this element is the one which we have identified as the inter- 

 medium in Tremcttops. It is similarly situated, and it has 

 between it and the fibulare a passage for vessels exactly as in 

 Trematops. The element situated distal to this intermedium is 

 manifestly the proximal centrale. The tibiale has remained, like 

 the majority of the other elements of the tarsus, cartilaginous. 

 The other ossified elements of the tarsus are figured as they occur. 

 I identified them as the first and fourth distal tarsals and one of 

 the centrales. Haughton, in describing the tarsus in the Bloem- 

 fontein specimen, regards the distal elements as the first and 

 third, but the Pretoria s]3ecimen has the element in undisturbed 



Text-figure 3. 



Text-fig. 3. — Right tarsus and metatarsus of Scincosaurus crassus Fritsch *. 

 After Jaekel. 



Text-fig;. 4. — Right tarsus and metatarsus oi Limnoscelis pahtdis Willistonf. 

 The positions of the missing elements, which were probably carti- 

 laginous, are indicated by letters. 



* A Microsaurian amphibian. 



t A primitive Cotylosaurian reptile. 



articulation with the head of the fourth metatarsal. Haughton 

 agrees in regarding the third small ossification as the centiale. 

 When the cartilaginous elements are restored, it will be seen 

 that the tarsus is almost exactly similar to that of Trematojjs. 



The most interesting feature of the Uranocentrodon tarsus 

 is the tendency that it exhibits of the elements on the tibial side 

 to become reduced. 



The next tarsus which we are able to study is that of the Micro- 

 saurian Scincosaurits crassus Fritsch. (text-fig. 3). Whether the 

 Microsauria are to be regarded as reptile-like amphibians or as 

 amphibian-like reptiles need not at present concern us, as it is 

 agreed by most that if they are amphibians they are apparently 

 the nearest to the pi'imitive reptiles. It is extremely fortunate 



Proc. ZooL. Soc— 1921, No. X. 10 



