ON THE STRUCTURE OF THE REPTILIAN TARSUS. 1-43 



6. On the Structure of. the Reptilian Tarsus. 

 By R. Broom, D.Sc, F.R.S., C.M.Z.S. 



[Received November 9, 1920 : Read February 22, 1921.J 

 (Text-figures 1-27.) 



In 1864 Gegenbaur established the general homology of the 

 elements of the tarsus with those of the carpus, and gave us a 

 series of names for the elements which have been used by most 

 later writers. 



The typical carpus in the higher forms was shown to be made 

 up of a radial e and an ulnare with an intermedium between them, 

 a centrale near the middle of the carpus, and five distal carpalia. 

 In the hind limb the tibia and fibula undoubtedly correspond 

 with the radius and ulna of the fore limb and the five distal tarsalia 

 as unquestionably agree with the five distal carpalia, but as the 

 proximal part of the tarsus usually has only three elements, there 

 has always remained some little doubt as to how to homologise 

 them with the four proximal carpals. 



Gegenbaur considered that the two proximal elements of the 

 mammalian tarsus — the calcaneum and astragalus — corresponded 

 with the ulnare, the intermedium, and radiale of the fore limb 

 and that they ought to be regarded as the fibulare and conjoined 

 intermedium and tibiale, while the third element, the navicular, 

 he looked upon as the centrale of the tarsus. As the inter- 

 medium and radiale are frequently united in mammals, it seems 

 very natural to conclude that the intermedium might be perma- 

 nently fused with the tibiale in the tarsus. 



Gegenbaur's view has been followed by the majority of later 

 comparative anatomists and palaeontologists. When it was 

 seen, however, that there was no evidence from either palaeon- 

 tology or from the study of the skeletogenesis in favour of the 

 astragalus being a composite element, most workers came to favour 

 the view that the astragalus is the tibiale alone and that the 

 intermedium has been early lost, though some few preferred to 

 look on the astragalus as the intermedium and to consider that 

 it was the tibiale that was lost. At the present time, though 

 the large majority of authorities support the former view, the 

 question is by no means settled, and I think there are good 

 reasons to believe that the generally accepted view is a mistaken 

 one. Within recent years, palaeontology has given us so much 

 new light that it seems necessary to reopen the question. 



If we had well-preserved tarsi of all the Carboniferous and 

 Permian amphibians and reptiles known, there would be no 

 difficulty in giving the complete evolutionary history of the 

 tarsus in its later stages. TJnfortunately, the tarsus of most of 

 the early Tetrapods remained largely cartilaginous, and even 

 where the elements are ossified it is rarely that we find them in 



