STRUCTURE OF THE REPTILIAN TARSUS. 



147 



placed in a distinct ordei^ — the Procoloplionia. The tarsus is known 

 with the elements in undisturbed position. There are two ossified 

 proximal elements and four distal tarsalia. The two proximal 

 elenjents have a passage between them, and are manifestly the 

 homologues of the large elements in Scincoscmrus and thus the 

 fibulare and intermedium. There has possibly been a cartila- 

 ginous tibiale, which is not preserved, as it seems necessary to 

 have an element to the tibial side of the intermedium to suppoi't 

 the first tarsale. Goodrich's figure of the Frocolo'plion tarsus, 

 which is taken from Watson's much-reduced restoration, gives 

 rather a misleading idea of the structure. In Frocolophon the 

 radiale in the carpus has evidently been cartilaginous, and it is 

 thus not at all remarkable that the corresponding tibiale in the 

 tarsus should also remain cartilaginous. 



When we follow the line of mammalian descent through the 

 American Pelycosaurs and allied forms and through the South 

 African Therapsids, we have as many well-preserved tarsi as we 

 require. 



Text-figui'e 5. 



Text-fie-ure 6. 



Text-figure 7. 





Text-fitr. 5. — Riglit tarsus and metatai'sus of JEosauravus copei Willistoii*. After 

 Williston. The distal tarsals are evidently somewhat displaced. The larg:e 

 oval element between the intermedium and the first metatars-al is probably 

 the displaced tibiale. The smaller element lying proximally to it is probabl}' 

 the first tarsal. The largest distal tarsal is probably the fourth tarsal. 



Text-fia;. 6. — Right tarsus and metatarsus of Seymouria haylorensis Broili f. After 

 Williston. 



The third and fourth tarsalia are lost. 



Text-fig'. 7. — Right tarsus and metatarsus of ProcoIopJicn irigoniceps Owen J. 

 From a specimen in the Alban\' Museum. The elements are preserved in 

 almost undisturbed relations. There was most probablj^ a small cartilaginous 

 tibiale in the position indicated by the letter "t." 



* A primitive Reptile of unknown affinity. 



t A primitive Cotylosaurian reptile. 



X A late Upper Triassic Cotylosaurian reptile 



The most primitive type we know is Ophiacodon minis Marsh 

 (text-fig. 8). Here there are two large proximal elements, mani- 

 festly those which become the calcaneum and astragalus of the 

 mammal, with five distal tarsals and two small elements lying distal 

 to the astragalus. The calcaneum and astragalus are manifestly 

 again the two elements which in lower types we have identified as 

 fibulare and intermeditnn ; and the five distal tarsals present no 



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