56 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I49 



others at hand and possibly began ossification from a separate center 

 before joining the scaphoid. 



Scaphoid. — The scaphoid is relatively shortened proximodistally 

 but noticeably elongate in a dorsoventral direction and exhibits a 

 knoblike ventromedial extremity. Displayed across the shorter dor- 

 sal portion of the proximal surface, the prominent articular convexity 

 or radial facet extends distad on the dorsal surface nearly to the 

 distal facet for the trapezoid. The distal surface is lunate in outline 

 and the dorsal or anterior portion shows an obliquely oriented con- 

 cavity for articulation with the trapezoid. Ventromedial to this there 

 is a more flattened to somewhat convex surface for the trapezium. 

 The distolateral angle of the dorsal surface is developed as a small 

 process which may be relatively prominent as discussed above. 

 Ventral from this point a rounded dorsoventral ridge articulates nar- 

 rowly with the magnum. This ridge divides the concave trapezoidal 

 area medial to it from the small, flattened, dorsally situated, and 

 laterally facing lunar facet immediately proximal to it. 



The Phenacodus scaphoid as well as being much deeper is dis- 

 tinctly less attenuated ventral to the radial and trapezoidal facets. 

 Moreover, the convexity for articulation with the radius does not 

 extend relatively so far down the dorsal surface as in Meniscotherium. 

 The facet for the trapezoid is comparatively flat, and that for the 

 lunar may be small and dorsodistally placed on the lateral surface. 

 There does not appear to be any contact with the lunar unless these 

 may touch at a point ventral to the lunar facet, but evidently not 

 near the dorsal surface of the wrist as in Meniscotherium. 



Lunar. — The lunar in Meniscotherium is an arcuate, proximodis- 

 tally compressed tonguelike structure. It laps dorsoventrally over 

 the proximal hump of the magnum, and its proximal convexity is 

 essentially concentric with its distal concavity. The essentially 

 convex facet for the lateral or lunar facet of the radius covers the 

 entire proximal surface and dorsally approaches the distal surface. 

 Ventral to the median transverse crest this surface is slightly concave 

 before reaching the ventral margin. The concave distal surface is 

 divided for articulation medially and somewhat more dorsally with 

 the magnum and laterally and more ventrally with the unciform. 

 The dorsal portion of the arcuate medial surface of the lunar is 

 faceted for articulation with the scaphoid. The ventral portion of 

 the proximodistally somewhat deeper or less arcuate lateral surface 

 shows a comparatively large facet for the unciform. 



The deep, more nodulelike Phenacodus lunar has a strongly con- 



