No. 3.] OSTEOLOGY OF MESOHIPPUS AND LEPTOMERYX. 325 



functional members, the second, third, and fourth, and one rudi- 

 mentary, the fifth. Metacarpal II. is very slender, compressed, 

 and elongated, with a much flattened shaft which is closely ap- 

 plied to that of the median digit, and which is arched forward. 

 The proximal end is somewhat thickened antero-posteriorly, but 

 very little wider than the shaft ; it exhibits three well-marked 

 facets, — a small one on the posterior or radial edge, for the 

 trapezium ; a larger, slightly concave, and truly proximal one for 

 the trapezoid ; and on the ulnar side a small facet for the mag- 

 num. The distal trochlea is very much compressed laterally 

 and narrower than the inferior end of the shaft ; the carina is 

 very nearly median in position, not at all prominent, and entirely 

 confined to the palmar side. 



Metacarpal III. is by far the largest of the series, though it is 

 relatively much narrower and more slender than in the later 

 genera of the equine phylum. The shaft is flattened antero- 

 posteriorly, and of transversely oval section ; it is narrowest 

 and deepest above, becoming broader and shallower distally. 

 The proximal surface is nearly plane, but is not placed so nearly 

 at a right angle with the long axis of the shaft as in the horse, 

 rising gently toward the ulnar side. This facet for the mag- 

 num is broader in front than behind, — a difference which is more 

 marked than in Eqiuis, and is suddenly constricted in the mid- 

 dle. The proximal surface displays no facets for the trapezoid 

 or unciform. The unciform facet is on the ulnar side of the 

 head and presents altogether laterally ; this facet is divided by 

 a deep groove into anterior and posterior portions. The distal 

 end is expanded laterally just above the trochlea, which is nar- 

 rower ; the carina is much stouter and more prominent than in 

 the lateral digits, and extends farther upon the anterior face, 

 but not so far as in Ancliithcrium. 



Metacarpal IV. is more slender than No. III. and has a nar- 

 rower and less thickened head. The unciform facet is nearly 

 plane and of triangular shape, with the apex directed posteriorly. 

 Upon the radial side there is a small facet for the overlapping 

 projection of metacarpal III., and on the ulnar side another for 

 the head of No. V. 



The fifth metacarpal is represented by a rudiment which 

 carries no phalanges. The head is as large as in Nfl. IV., but 

 the shaft is very slender and tapers rapidly to a point. The 



