NO. 2 A STUDY OF MENISCOTHERIUM GAZIN 63 



of the articular surface has less diameter and its transverse axis is 

 slightly oblique to the long axis of the shaft, the asymmetry being in 

 reverse of that characterizing the second metacarpal. 



In Phenacodus the proximal surface is relatively broader dor- 

 sally, and the lateral margin of surface for the unciform does not 

 extend so noticeably distal on the dorsal surface. Moreover, the 

 facet for the third metacarpal appears more sharply deflected from 

 the unciform surface, and that for the fifth metacarpal faces more 

 laterally than in M eniscotherium. 



Metacarpal V. — The fifth metacarpal is approximately three-quar- 

 ters the length of the fourth with a more slender and compressed 

 shaft but with moderately large extremities. The dorsal surface of 

 the bone is nearly straight so that the ventral surface appears dis- 

 tinctly concave longitudinally. This element in M eniscotherium is a 

 little longer and distinctly more robust than the first metacarpal and 

 is not so noticeably bowed. The proximal extremity is broad but not 

 deep dorsoventrally. The surface for the unciform is strongly convex 

 dorsoventrally, and transversely it is of nearly uniform width and 

 straight or only slightly convex. Sharply deflected distally from the 

 inner or medial margin of the unciform facet is an elongate, cres- 

 centic surface extending medially and distally for articulation with the 

 fourth metacarpal. The distal extremity is a little smaller than that 

 of the fourth but the form is nearly identical. Its asymmetry is not 

 so striking as in the first metacarpal. 



In the Phenacodus skeleton that Cope described, the fifth meta- 

 carpal is about the same length as the first but very much sturdier. 

 In another articulated fore foot (A.M. No. 2961) the fifth metacarpal 

 is much longer as well as heavier than the first. In either case the 

 fifth is shorter relative to the fourth than in M eniscotherium. While 

 short in comparison with M eniscotherium, the Phenacodus fifth 

 metacarpal has a relatively much greater diameter. In an instance 

 where the ratio of lengths is about two to one, diameters of the shaft 

 are between three and four to one. The proximal extremity of the 

 fifth metacarpal is deeper dorsoventrally in comparison with its width 

 than in M eniscotherium and the facet for the fourth metacarpal is 

 relatively smaller and does not extend distomedially so noticeably. 

 Both exhibit a strong process or tuberosity on the lateral surface of 

 the proximal extremity for the extensor carpi ulnaris. 



Phalanges. — The proximal phalanges of the M eniscotherium 

 manus are approximately half the length of the corresponding meta- 

 carpals. They are slender and of nearly uniform width, tapering only 



