NO. 2 A STUDY OF MENISCOTHERIUM GAZIN 6l 



relatively more robust through the shaft than in Meniscotherium 

 and is distinctly less bowed or arched. Proximally it evidently also 

 articulated only with the trapezium. 



Metacarpal II. — The second metacarpal of Meniscotherium is next 

 to the third metacarpal in length and only slightly shorter. The 

 greater part of the nearly triangular proximal surface articulates 

 with the trapezoid. This facet is slightly convex dorsoventrally but 

 distinctly concave transversely. Its lateral margin is a low ridge 

 which divides the trapezoidal surface from a narrow somewhat dis- 

 tally deflected facet for the magnum. Distal to this on the lateral sur- 

 face of the proximal extremity is a deep concavity with a somewhat 

 distally facing dorsoventrally concave facet for the proximal extremity 

 of the third metacarpal as the base of the second metacarpal over- 

 rides it. The medial side of the proximal extremity of the second 

 metacarpal shows an arcuate facet along the proximodorsal margin 

 for the trapezium. The shaft of the metacarpal is essentially straight 

 and dorsoventrally somewhat flattened, but its width narrows only 

 slightly from the base, then expands to the distal extremity which is 

 a little wider transversely than the proximal portion. 



The distal extremity shows an evenly rounded articular surface for 

 the proximal phalanx, with a distinct keel only on the ventral half. 

 The transverse axis of this convexity is slightly oblique to the long axis 

 of the shaft. 



The proximal extremity of the more robust Phenacodus second 

 metacarpal is rather similar to that of Meniscotherium, although the 

 trapezoidal facet is perhaps somewhat flatter and more nearly rec- 

 tangular, and that for the trapezium of relatively smaller area. The 

 shaft of the metacarpal, moreover, is not so slender and appears 

 much less compressed dorsoventrally. The length of this bone in 

 Phenacodus primaevus is at least two and one-half times that in 

 Meniscotherium chamense, whereas the length of the humerus is a 

 little less than twice that of the latter. 



Metacarpal III. — Proximally the base of the third metacarpal in 

 Meniscotherium has the form of a truncated triangle or trapezial 

 outline, somewhat more so than that of the second metacarpal. The 

 surface for the magnum is decidedly convex dorsoventrally but 

 nearly straight transversely, sloping distally toward the medial margin 

 and grading almost imperceptibly into a relatively broad marginal 

 surface for the distally facing facet on the lateral side of the second 

 metacarpal. The dorsal two-thirds of the lateral margin of the base 

 of the third metacarpal exhibits a crescentic, nearly lateral facing 



