60 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I49 



concavity of the magnum, where there seems to be little or no contact 

 in Menisc other ium. 



Unciform. — The dorsal face of the Meniscotherium unciform 

 appears of nearly uniform depth because of the distally extending lip 

 on the lateral half. In ventral view, however, the bone is decidedly 

 triangular with the sloping proximal surface for the cuneiform reach- 

 ing in a thin lateral margin the facet for the fifth metacarpal. A 

 dorsoventrally convex and transversely narrow facet for the lunar 

 is deflected more medially from the proximomedial margin of the 

 broad surface for the cuneiform. The distal surface is dorsoventrally 

 concave, and a low saddle separates the larger medial surface for the 

 fourth metacarpal from the smaller surface for the base of the fifth 

 metacarpal. The proximal two-thirds of the nearly flat dorsal por- 

 tion of the deep medial surface articulates with the magnum. The 

 distal margin of this surface articulates with the broad base of the 

 third metacarpal. 



The Phenacodus unciform is relatively much deeper proximodistally, 

 and although there is some lateral convergence of the dorsoventrally 

 more convex surface for the cuneiform with the concave surface 

 for the fifth metacarpal, the lateral margins of these surfaces appear 

 well separated. It should be noted, moreover, that proximomedially 

 the unciform articulated with the magnum, somewhat as in Menisco- 

 therium, but with relatively narrower contact, although Cope saw no 

 facet for this on the unciform. Presumably that for the magnum 

 blended too smoothly with the surface for the cuneiform in the speci- 

 men he examined. The facet is particularly evident on the magnum. 

 As in Meniscotherium much of the dorsal portion of the medial 

 surface articulates with the lunar and the dorsodistal part of this 

 surface with the third metacarpal. 



Metacarpal I. — The first metacarpal is somewhat reduced in Menis- 

 cotherium; about half the length of the third and with a relatively 

 more slender and dorsally bowed shaft. The proximal extremity is 

 noticeably enlarged with a convex articular surface for the trapezium 

 which is elongated corresponding to the broad dimension of the 

 trapezium's distal surface. I see no evidence for articulation with the 

 second metacarpal. The distal extremity is enlarged but to a some- 

 what less extent with a transversely narrow and medially tapering 

 articular surface for the first phalanx. The subdued keel appears 

 displaced toward the lateral side. 



The Phenacodus first metacarpal is evidently somewhat variable 

 in length in comparison with the other metacarpals but appears 



