62 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 149 



facet for the unciform, and distal to this, much as in the second meta- 

 carpal, there is a lateral concavity with a dorsoventrally concave, 

 essentially distal facing facet accommodating a portion of the proxi- 

 mal surface of the fourth metacarpal, overriding it much as the second 

 overrides the third. The shaft of the third metacarpal is much like 

 that of the second but is a little longer and shows, moreover, a 

 noticeable scarlike prominence on the medial side of the dorsal sur- 

 face about a quarter of the length distally from the base, evidently 

 for insertion of the extensor carpi radialis brevis. The distal articular 

 surface for the proximal phalanx is similar to that of the second 

 metacarpal but more nearly symmetrical, and the transverse axis is 

 perpendicular to the long axis of the shaft. 



The proximal extremity of the Phenacodus third metacarpal is 

 much like that of Meniscotherium except that the surface for the 

 second metacarpal is much more restricted and turns sharply distal 

 with respect to that for the magnum. The lateral surface of the 

 proximal extremity differs from that in Meniscotherium in that the 

 facet for the fourth metacarpal is deflected somewhat more laterally. 

 The shaft of the bone is relatively not so slender and is distinctly less 

 compressed dorsoventrally. 



Metacarpal IV. — The fourth metacarpal is appreciably shorter than 

 the second, but because of the successively overlapping proximal 

 extremities the distal extremities in the articulated foot are about 

 even. The proximal surface of the Meniscotherium fourth meta- 

 carpal is more nearly triangular than on the second metacarpal. 

 This surface is decidedly convex dorsoventrally but only slightly so 

 transversely. Much of the triangular surface to the lateral margin 

 articulates with the unciform and projects distally onto the lateral 

 margin of the dorsal surface to a noticeable extent, corresponding 

 to the extension of the dorsal lip of the unciform in its lateral part. 

 Distal to the lateral margin of the proximal surface there is a con- 

 cavity, not so deep as in the second and third metacarpals, and the 

 facet for the fifth metacarpal is strongly arcuate, has a marked dorsal 

 extension, but faces more laterally than in the second and third. 

 The medial margin of the proximal surface is slightly offset distally 

 in its dorsal portion for the overriding margin of the third metacarpal. 

 The ventral portion of this surface, however, appears to be nearly 

 separate, and is more oblique and may project proximally somewhat 

 from the unciform facet. The shaft of the fourth metacarpal is 

 relatively straight and dorsoventrally flattened as in the others. Its 

 distal extremity shows a slight asymmetry in which the lateral half 



