58 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 49 



about equaling the greater diameter of the cuneiform. Its articular 

 surfaces are transversely broad but proximodistally shallow, although 

 both increase somewhat in depth laterally. The surface for the ulna 

 is longitudinally concave near the lateral extremity and makes a 

 sharp angle with the more flattened facet for the cuneiform. The 

 shaft of the pisiform is broad proximally but transversely constricted 

 along it's distomedial margin. The posterior extremity of the pisiform 

 exhibits an enlarged knob. 



In Phenacodus the articular extremity of the large pisiform is 

 relatively not so broad transversely, the articular facets are proxi- 

 modistally deeper, and there is much less of an angle between them 

 than in M eniscotherium. The facet for the cuneiform is decidedly 

 triangular, articulating well down on the ventrolateral projection of 

 the cuneiform. The shaft of the pisiform is relatively much deeper 

 proximodistally than in M eniscotherium and appears comparatively 

 narrow. The shaft is elongate and terminates posteriorly in a simi- 

 larly enlarged tuberosity or knob. 



Trapezium. — The trapezium is broad and proximodistally deep in 

 M eniscotherium, and its medial facing surface is comparatively flat. 

 The proximal portion narrows somewhat toward its articulation 

 with the scaphoid, but more distally the dorsal margin is dorsally 

 expanded or flaring, although the ventral margin is nearly straight. 

 The distal extremity exhibits an oval-shaped, slightly concave facet 

 for the first metacarpal. The lateral facing surface has a distinct 

 prominence proximally which supports a narrow but transversely 

 elongate facet on its dorsolateral surface for articulation with the 

 trapezoid, and just below or distal to this a somewhat arcuate surface 

 extends dorsodistally out on the dorsal flare of the trapezium for 

 contact with the base of the second metacarpal. 



The Phenacodus trapezium appears more nearly rectangular and 

 relatively thicker transversely along the ventral margin. The proxi- 

 mal articular surface for the scaphoid is convex, and that distally 

 for the first metacarpal is concave and somewhat larger. The lateral 

 surface shows pairs of facets proximally and distally for the trapezoid 

 and second metacarpal respectively. 



Trapezoid. — The trapezoid in M eniscotherium is noticeably com- 

 pressed proximodistally and nearly wedge-shaped with the deepest 

 part at the dorsomedial angle of the bone. The dorsal surface exhib- 

 its a prominence distally near the medial margin. The proximal 

 surface for the scaphoid is dorsoventrally convex in the dorsal part 

 but changes to concave in the ventral portion. The distal facet for 



