158 



University of California Publications. [Geology 



The trapezoid (PI. 20, figs. 7-8 t) is a small element, tri- 

 angular in section when viewed from below. The facet for 

 articulation with the scapho-trapezius which occupies the entire 

 proximal surface is convex in dorso-palmar section and sigmoid 

 transversely. The articulation is continued on the inner side 

 of the bone as a surface inseparable from the proximal one. An 

 oblong dorsal facet, concave in dorso-palmar section and a small 

 ellipsoidal convex palmar facet, separated from the former by 

 a groove, articulate with the magnum. A large distal facet, 

 sigmoid in dorso-palmar section, articulates with the second 

 metacarpal. 



The magnum (PI. 20, figs. 7-8 m) is broken, and has lost the 

 articular surfaces for the scapho-trapezius and the lunar. 

 There is an irregularly oval palmar facet for the scapho- 

 trapezius, a concave roughly quadrilateral distal facet for the 

 third metacarpal and two facets for the unciforme, a large 

 irregularly elongate facet proximally, and a small acutely 

 ovoidal facet on the anterior distal edge of the bone. 



The calcaneum (PI. 22, figs. 2 and 3) differs from the cor- 

 responding element in Megalonyx jeffersonii in the following 

 respects : 



(a) The postero-inferior extremity of the tuber calcis is 

 directed toward the inner side, while in jeffersonii it is directed 

 toward the outer side.* 



(b) The inner surface of the tuber calcis is concave antero- 

 posteriorly, while in the vertical direction it is convex below and 

 plane or slightly concave above. In jeffersonii, it is concave in 

 both directions. 



(c) In jeffersonii, "posterior to the articular extremity, the 

 calcaneum forms a large plate nine inches in depth, and only a 

 fourth of an inch in thickness toward the center, "f In the 

 Californian species, this plate exceeds three-quarters of an inch 

 in thickness toward the center and is never less than half an inch 

 in thickness at the margin. 



(d) While the neck of the tuber, behind the articular sur- 

 faces is only slightly wider vertically, than in jeffersonii, the 



* J. Leidy, op. fit., PI. XII, fig. 6. 

 t Lor. cit. p. 41. 



