104 THE MAMMALIA OF THE DEEP KIVER BEDS. 



seen from the following points of difference : (1) In the European species the shaft 

 is less broad and flat, more slender and rounded; (2) the bicipital tubercle is on the 

 internal face; (3) the proximal end is less expanded; (4) the distal end is narrower 

 and more distinctly trihedral ; (5) the carpal surfaces have less antero-posterior 

 extension, and, in particular, the scaphoid surface extends less behind the plane of 

 the lunar. In this respect, the carpal facets of A. aurelianense resemble more those 

 of HJquHS asinus, while A. equinum approximates JE. caballus. 



The Carpus (PI. IV, Fig. 31). The proportions of the scaphoid are very similar 

 to those seen in the horse, it being only slightly narrower and higher in relation to 

 its depth fore and aft. The proximal surface, however, differs from the condition 

 found in the modern genus in a way corresponding to what has already been described 

 in the radius, viz., in the greater convexity of the anterior portion and narrowness in 

 the palmar part. Distally, the differences are more important, as is seen in the much 

 less relative size of the magnum facet and the narrower and more deeply concave 

 surface for the trapezoid, as well as in the presence of a distinct facet for the trape- 

 zium, which is absent in the horse. The facets for the lunar are smaller and less 

 projecting than in the latter. The scaphoid of the European species is almost pre- 

 cisely the counterpart of that of the American form ; the antero-external angle is 

 somewhat more elevated and the distal facets have a slightly different shape. 



The lunar differs from that of JEquus much more than does the scaphoid; it is 

 both higher and narrower, and the proximal surface especially has smaller propor- 

 tionate diameters, both transversely and antero-posteriorly. The radial facet diners 

 in being quadrate rather than triangular ; it is much less extended on the palmar 

 side and lacks the additional facet on the posterior crest which occurs in the recent 

 animal ; in front, the descent towards the radial side is both greater and more abrupt ; 

 the lateral facets for the scaphoid and cuneiform are much less prominent, and hence 

 the median constriction of the lunar, when viewed from the front, is not nearly so 

 marked. Distally, we note that the unciform facet is wider and more oblique and 

 that for the magnum is smaller in both dimensions ; this is especially true of its pos- 

 terior prolongation, the well-developed, knob-like process projecting considerably 

 beyond it, as it does not in the horse. The lunar of the European species differs 

 from that of the American principally in the greater breadth of the facet for the 

 unciform and the less antero-posterior extent of that for the magnum. 



The cuneiform has a much greater antero-posterior, as compared with its verti- 

 cal diameter, than is the case in the horse ; it is also narrower transversely and more 

 compressed. The principal pisiform facet is much smaller and is not isolated, as in 

 Equus, but is connected with the ulnar facet by a narrow articular surface. The 



