1918] Stock: The Pleistocene Fauna, of Hawver Cave 



505 



internally such a distinct boss or knob as in Megalonyx. This is due 

 in part to the fact that the articulating surface slopes away more 

 gradually from the top of the knob to the dorsal margin of the unci- 

 form. In Megalonyx- a shelf is formed at the base of the knob and 

 the end of this shelf is the dorsal margin. The proximal knob differs 

 further from Megalonyx and Hapalops in being curiously flattened 

 along the inner side. On the inner side the facet for the magnum 

 resembles that in Megalonyx and is relatively less extensive than that 

 in Hapalops. Dorsally a small facet for the magnum, continuous 

 distally with the facet for metacarpal 3, is not connected with the 

 proximal articulating surface for the magnum as in the unciform, 

 no. 22775, of Megalonyx from Rancho La Brea. The facet for meta- 

 carpal 3 (fig. 25(7) is wide dorsally and narrows decidedly to the 

 ventral side, thus resembling* the corresponding feature in Megalonyx 

 jeffersoni as figured by Leidy, 20 and differing from that in Mega- 

 lonyx from Rancho La Brea. The widest portion of the facet differs 

 from that in the latter genus in being deeply excavated, especially 

 in an oblique line extending from the inner to the dorsal margin. 

 In the unciform of Hapalops, the oblique groove traversing the facet 

 dorsally is very much deeper than in Nothrotherium and notches the 

 distal border of the carpal element. This deep groove is to be asso- 

 ciated with a more extensive overlapping of metacarpal 3 upon the 

 fourth metacarpal. In Hapalops the unciform joins with the external 

 and internal sides of the overlapping processes, while in Nothro- 

 therium articulation is with the external or true proximal face of 

 the metacarpal. 



In Nothrotherium the facet for metacarpal 4 differs from the cor- 

 responding surface in Hapalops in its much smaller size but resembles 

 the latter in being deeply excavated. The facet in Nothrotherium 

 is comparable in size to that in M. jeffersoni, but is decidedly smaller 

 than in no. 22775 from Rancho La Brea. A small articulating facet, 

 also for metacarpal 4, truncates the proximal portion of the boundary 

 between facets for the third and fourth metacarpals. 



The dorsal surface of the unciform (fig. 25a) differs from that 

 in Megalonyx in being wider proximo-distally along the inner side, 

 and the dorso-internal corner is not as prominent. 



A second metacarpal, no. 19872, is very poorly preserved. The 

 proximal end of this specimen is fragmentary and shows only portions 

 of the articulating surfaces for trapezoid and trapezium. The former 



2° Leidy, J., A memoir on the extinct sloth tribe of North America, Smithson. 

 Contrib. Knowl., vol. 7, pi. 8, fig. 13b, 1855. 



