326 SCOTT. [Vol. V. 



unciform facet is narrow and slightly concave from side to side, 

 the ulnar border rising somewhat higher than the radial. 



The metacarpus of Anchitherium exhibits no very important 

 differences from that of MesoJiippus. The median digit is much 

 broader relatively and appears also to be longer in proportion ; 

 the unciform facet is more obliquely placed, so as to present 

 upward as well as laterally ; the carina of the distal trochlea is 

 somewhat more prominent, and continued farther up upon the 

 anterior face of the bone. The second and fourth metacarpals 

 are very much as in the American genus, but judging from the 

 facet upon the unciform, metacarpal V. is still further reduced, 

 and must have been concealed from view by No. IV., when seen 

 from the front ; in MesoJiippus it is plainly visible in the ante- 

 rior aspect. The carpo-metacarpal articulations have undergone 

 no important modification from the condition found in the latter. 



The metacarpus of the horse (see Fig. B, p. 336), on the other 

 hand, presents some very important changes. The fifth digit 

 has entirely disappeared, the second and fourth reduced to mere 

 splint bones without phalanges. The median metacarpal has 

 greatly increased in breadth and thickness, especially toward 

 the radial side, and the distal carina has become very stout and 

 encircles the entire trochlea, dorsal as well as palmar. The 

 anterior unciform facet has become almost entirely proximal 

 instead of lateral in position. Seen from the front, the carpo- 

 metacarpal articulations seem to have undergone little change, 

 but the great increase in breadth of the posterior portion of the 

 head has brought the median metacarpal into contact with the 

 trapezoid and unciform, thus supporting it on all of the distal 

 carpals. 



The phalanges of MesoJiippus are very like those of AncJiitJie- 

 rium. Those of the lateral digits are short, slender, and much 

 compressed laterally ; the unguals narrow and pointed. Those of 

 the median digit are longer, very much broader, and more sym- 

 metrical in shape. The proximal one is relatively much shorter 

 than in the horse ; the groove for the metacarpal carina is con- 

 fined to the palmar half, and the distal articular surface does 

 not extend so far upon the dorsal side. The ungual phalanx is 

 long, broad, flattened from above downwards, and pointed. It 

 is not so much elongated as in AncJiitJicrium, but very different 

 in shape from the characteristic ungual of the existing Equid<2. 



