154 



DR. E. BROOM ON THE 



of its being made up of a fibulare, an intermedium, a tibiale, and 

 a centrale. I tbink it more probable that there are only in it the 

 fibulare, intermedium, and tibiale. The evidence for a centrale 

 is not at all satisfactory. If, however, it is really the conjoined 

 fibulare, intermedium, and tibiale, then the whole tarsus becomes 

 strikingly similar to that of Hoivesia. 



I give a figure of the tarsus of a very young embryo of 

 Testudo sp. (text-fig. 23). Here there are only seen two 

 proximal elements and four distal tarsals. There is no evidence of 

 the larger proximal elements being more than a single element, 

 and I am inclined to regard it as intermedium alone. Other 

 Chelonians are known to have an additional element between the 

 larger proximal element and the first and second tarsals. 



Text-figure 24. 



Text-figure 25. 



Text-fig. 24. — Right tarsus and metatarsus of Plesioscmrus rugosus. After Owen. 



Text-fig. 25. — Right tarsus and metatarsus of Peloneustes jphilarclius Seeley. 

 After Andrews. 



Goodrich considers this to be the centrale. I regard it as the 

 much reduced and displaced tibiale. The fifth metatarsal is 

 shortened up. 



In most of the later Diapsidans we find a tarsus which is 

 either of the Sphenodon type or a modification of it. 



In the aquatic reptiles we find many interesting types. 

 Ichthyosaurus is too specialized to be of much morphological 

 interest, but the Triassic form, Mixosaurus, shows us how the 

 Ichthyosaurian paddle has originated. I believe the tarsus to 

 be made up of fibulare and intermedium with the five distal 

 tarsals, and to be thus almost identical with the tarsus of 

 Mesosaurus or Stereo-sternum, which there is some reason to 

 consider as perhaps its nearest allies. 



