I'l I KKSON : OSTKOI.OOY OF OXVDACI VLUS. 459 



tion. The two bones are closely applied to each other with rough- 

 ened surfaces throughout nearly their entire length. Distally they are 

 slightly separated. Metacarpal III is proportionally heavier than that 

 of the llama. Proximally the head rises above that of metacarpal IV, 

 and carries four facets for articulation with the unciform, magnum, 

 trapezoid and trapezium, respectively. The magnum facet is slightly 

 concave and occupies the anterior face, while the posterior facet for 

 the trapezoid is more elevated and extends downward on the posterior 

 face of the bone. The decurved part of the latter facet, although 

 without a distinct separation from that for the trapezoid, serves to ar- 

 ticulate with the ovate-shaped facet referred to as present on the tra- 

 pezium. On the ulnar side, near the anterior face, there is a process 

 with a rounded articular face, which abuts against a corresponding 

 facet on the unciform. Immediately below this process is the articu- 

 lation for metacarpal IV. The radial face of metacarpal III is com- 

 paratively flat and broad, the anterior narrow and very convex. The 

 ulnar face is flat, rough, and is closely applied to metacarpal IV. 

 Posteriorly the two metacarpals have prominent lateral ridges, thus 

 forming a wide, deep groove in the median line, which extends from 

 the proximal end down the shaft two thirds of its length. The lateral 

 posterior ridge is much more pronounced on metacarpal III than 

 metacarpal I\'. In the recent camel and llama the posterior ridges 

 on the metacarpals are more nearly equal in size and the median de- 

 pression is comparatively shallow. Half way down on the shafts of 

 the metacarpals of O. loi}i:;ipes the cross-sections gradually take on a 

 D-shaped form which is continued to the distal trochlea. In the early 

 forms referred to the Tylopoda the distal trochlea is very narrow, a 

 character which is retained in O. longipcs, the trochlea being scarcely 

 any wider than the shaft of the bone. In the recent camel and llama 

 the trochlea is wide, but the carina is much stronger in O. lon^^i/^es 

 than in these genera. Metacarpal IV is lighter, slightly shorter and 

 more angular in cross-section near the proximal end than metacarpal 

 III. The ])roximal end has a single articular facet for the unciform. 

 Another small facet is seen on the antero-radial side for articulation 

 with metacarpal III. The shaft has less depth antero-posteriorly than 

 that of the third metacarpal. The size of the distal end, including the 

 trochlea, is more nearly equal to that of metacarpal III than is the 

 proximal end of the bone, and the phalanges of the two digits are 

 about equal in size. 



