PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 



(17) The patella is almond-shaped and of no great thickness. 

 ' (18) The tibia and fibula are never coossified. The tibia is short and 

 heavy, but much less so than in the Pleistocene genera; the proximal 

 end is antero-posteriorly compressed and has widely separated facets for 

 the femoral condyles; the shaft is arched forward and the cnemial 

 process, though heavy and rugose, is inconspicuous; the distal end is 

 broad and much compressed antero-posteriorly; the inner malleolus is 



Pes of Santa Cruz Gravigrada, x 

 c, Pelecyodon arcuatus. 



IV 



FIG. 26. 



a, Hapalops longiceps ; b, Analcimorphus leptocephalus ; 



very small, but internally to it there arises a prominent process, which 

 is grooved by a tendinal sulcus ; the astragalar surface varies consider- 

 ably in the different families and genera. 



(19) The fibula is stout and straight, with subcylindrical shaft and 

 extremely massive distal end, which has no contact with the calcaneum. 



(20) The pes, though known in only a few genera, displays consider- 

 able variety of structure. In the Megalony chides the astragalus is essen- 

 tially similar to that of the Pleistocene members of the family ; it is rela- 

 tively narrow and elongate, with trochlea which may be but slightly or 

 quite distinctly grooved ; the neck is very short and the navicular facet is 

 concave. In the Planopsidcg, as represented by Prepotherium, the astrag- 

 alus is of quite a different type, being shorter and much broader and with 

 quite deeply grooved trochlea ; the inner condyle is short and strongly 

 convex and projects prominently toward the internal side, thus displaying 

 in an incipient stage the odontoid form of this condyle seen in Mega- 

 therium and Mylodon. 



