54 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 149 



The distal extremity is enlarged and shows two nearly circular, 

 slightly concave facets side by side, the lateral or lunal somewhat 

 the larger, anteroposteriorly. The medial facet for the scaphoid, 

 however, extends a short distance medially over the anteroposterior 

 crest of the styloid process. 



The radius in Phenacodus resembles that of Meniscotherium in 

 the character of the proximal articular surface for the humerus but 

 appears relatively a little deeper anteroposteriorly. However, the 

 shaft is relatively much more sturdy, so that the head does not seem 

 to be so expanded. Distally the shaft is more rounded with less 

 prominent crests than in Meniscotherium, and as noted by Cope, the 

 distal portion close to the extremity is enlarged and converges toward 

 the facets for the carpus. 



ULNA 



The Meniscotherium ulna (see pi. 7) is decidedly flattened in an 

 anterolateral-posteromedial direction, and a nearly uniform width is 

 maintained from just distal to the sigmoid notch nearly to the distal 

 extremity. The sigmoid notch is markedly convex transversely and, 

 in keeping with the form of the trochlea of the humerus, shows 

 flaring proximolateral and distomedial margins. The olecranon proc- 

 ess is anteroposteriorly deep and elongate and shows a rather broad 

 posterior margin, widening proximally and curving somewhat forward 

 to a rugged medially deflected crest for insertions of the triceps 

 group or extensors of the forearm. On the anteromedial surface 

 of the shaft, just distal to the coronoid process there is a deeply 

 impressed and proximodistally somewhat elongate pocket or fossa, 

 presumably for the tendon of the brachialis and clavobrachialis mus- 

 cles. On the posteroexternal surface of the shaft a prominent median 

 crest arises a short way distal to the sigmoid notch and extends dis- 

 tally toward the posteroexternal margin where it abruptly widens or 

 divides, as noted in M. chamense. In the more robust form, how- 

 ever, the broadening of this crest commences more proximally with 

 the anterior margin continuing medially on the shaft. This crest 

 evidently bounds posteriorly the area of attachment or origin for the 

 extensor brevis policis. Anterior to the crest the shaft of the ulna is 

 noticeably concave anteroposteriorly. Distally the anteromedial mar- 

 gin of the shaft widens or divides with included space slightly rugose 

 for the interosseous membrane. The distal extremity is broad and 

 usually somewhat flattened, much as the shaft in general. The distal 

 articular surface for the cuneiform is variable but approximately 



