EDENTATA OF THE SANTA CRUZ BEDS. 



175 



In the Megalonychidce the calcaneum resembles that of Megalonyx and 

 Notkrotherium, especially in the broad, depressed, curved and plate-like 

 tuber; the sustentaculum is not very prominent and its facet for the 

 astragalus presents almost distally, so as to be nearly parallel with the 

 surface for the cuboid ; the latter surface is quite concave and of subcir- 

 cular shape, covering the entire dorso-plantar diameter of the distal end. 

 In Prepotherium the calcaneum displays some significant differences ; the 

 tuber is not notably different, being a similar broad, depressed and curved 

 plate, but the sustentaculum is very much more prominent and its facet for 

 the astragalus presents dorsally, at right angles to the plane of the cuboid 

 facet ; the latter is remarkably narrow in the dorso-plantar dimension and 

 below it is a thickened mass of bone. Doubtless, these differences indi- 

 cate that Prepotheritim had already begun to rest the weight in walking 

 upon the fibular border of the hind-foot. 



FIG. 27. 

 Dorsal side of right astragalus, nat. size, a, Hapcdops elongatus ; b, Prepotherium potens. 



The remainder of the pes is known in only a few genera of the Mega- 

 lonychidce, such as Hapalops, Analcimorphus and Pelecyodon, and shows an 

 unexpected amount of difference between them, which, however, is chiefly 

 confined to the relative proportions of the digits. The tarsal bones are 

 all present and unankylosed and the digits number five, all functional and 

 bearing claws. In Hapalops the three radial metatarsals (I-IH) are very 

 short, and II and III are extremely heavy, while IV and V are much more 

 slender and elongate ; V gives off from the proximal end of the fibular 

 border a very large and prominent process, as in Megalonyx, in which 



