No. 3 • ] OSTEOL OGY OF MESOHIPPUS AND LEPTOMER YX. 321 



is quite broad, the increase being chiefly upon the external side. 

 There are three clearly demarcated facets for the humerus, of 

 which the internal is the broadest, and is separated from the 

 external by the broad and deep intercondylar notch, which, how- 

 ever, emarginates the anterior edge but slightly. The external 

 facet projects strongly beyond the line of the shaft ; it is nearly 

 flat and descends steeply from behind forward, somewhat as in 

 Oreodon. The very unusual amount of flexion which this ar- 

 rangement permits has been already mentioned. Distally the 

 radius is thickened, but not much expanded transversely ; the 

 anterior face has broad, not very deep sulci for the extensor 

 tendons, and on the external side there is a deep groove for the 

 distal end of the ulna. There are two carpal facets, for the 

 scaphoid and the lunar ; the former is concave in front, and be- 

 hind forms a stout convex projection which extends much pos- 

 teriorly to the lunar facet and is reflected up upon the hinder 

 face of the bone. The surface for the lunar is simply oval and 

 concave ; its greatest diameter is transverse, in which dimen- 

 sion it somewhat exceeds the scaphoid, but is much less ex- 

 tended antero-posteriorly. The tubercle for the insertion of the 

 biceps and brachialis internus muscles is on the inner side of 

 the proximal end. The ulna and radius of Anchitherium present 

 no important differences from those of the American genus. 



In Equus the ulna and radius have completely coalesced and 

 the shaft of the former is interrupted. The olecranon is longer 

 and projects more strongly backward than in Mesohippus, and 

 its free end is more regularly rounded. The shaft of the radius 

 is much stouter and more expanded at the extremities ; the 

 facet for the internal humeral condyle is decidedly broader in 

 proportion, and the intercondylar groove notches the anterior 

 margin quite deeply. The rugosity for the biceps and brachialis 

 internus is on the anterior face. 



The carpus of Mesohippus (PI. XXIII., Figs. 26-28) is in 

 general appearance and construction very much like that of 

 the horse, but with many differences of detail, and on the 

 whole decidedly more primitive. The carpus of Anchitherium 

 is distinctly nearer to the modern condition, which is partly 

 due to its greater size, partly to the more pronounced enlarge- 

 ment of the median digit, and reduction of the lateral ones. 



The scaphoid is rather narrow transversely, but high and of 



