THE MAMMALIA OF THE DEEP RIVER BEDS. 107 



anterior facet for mc. ii presents more laterally and less posteriorly. The unciform 

 facet is single, but the deep sulcus, which in HJquus divides it into two parts, is pres- 

 ent in an incipient stage ; this facet is still entirely lateral and but little oblique in 

 position, while in the horse it has become altogether proximal. The facet for mc. ii 

 is double, but that for mc. iv still remains single, though showing a tendency to 

 divide into two parts. A striking difference between the two genera in regard to the 

 head of the median metacarpal lies in the very much narrower posterior portion in 

 the fossil. In the horse this region is extended beneath the unciform, on the one 

 hand, and the trapezoid on the other, and is separated, on the ulnar side, by a deep 

 sulcus from the anterior portion of the magnum facet. In A. equinum this posterior 

 region does not touch the trapezoid, its contact with the unciform is lateral, and the 

 sulcus which invades the magnum facet is only incipient. The shaft is of very uni- 

 form dimensions, contracting very little below the head, where it is of transversely 

 oval section ; for most of its length it retains much the same form and size, but 

 towards the distal end it gradually widens and becomes more flattened. The distal 

 trochlea is low and narrower than the shaft, which is broadened just above the troch- 

 lea by the tubercles for the attachment of the lateral ligaments. The carina is promi- 

 nent upon the posterior surface but dies away upon the anterior. In A. aurellanense 

 the specimens differ considerably in the character of the median metacarpal. In 

 those from Sansan the bone is very much as in the American species, but the facet 

 for mc. iv is double and the distal keel extends farther upon the anterior face. In 

 the specimens from Steinheim the keel is entirely restricted to the palmar side, but 

 the proximal end is quite modernized by the rounding of the anterior border of the 

 magnum surface, the widening of the posterior extension of this facet, and the par- 

 tial separation of the two by a sulcus from the ulnar side. 



The fourth metacarpal has a narrow, slightly concave head for the unciform, and 

 the posterior side displays a small facet for the proximal end of the rudimentary 

 mc. v. The shaft is truncated obliquely for about an inch below the head and forms 

 a roughened surface, to which the styliform rudiment was no doubt closely applied. 

 The shaft and distal end of mc. iv are the counterparts of mc. ii. A rudiment of 

 mc. v was also present in the European species. 



The Phalanges (PL III, Figs. 27, 28; PL IV, Fig. 31). In the lateral digits 

 the phalanges are different in many details from those of the French specimens of A. 

 aurelianense. The proximal phalanx is shorter in proportion to that of the median 

 digit, though of the same actual size, and relatively to the length of the metacarpals ; 

 beneath the proximal thickening the bone is more suddenly constricted and then 

 thickens again slightly to form the distal trochlea. The lateral profile is thus very 



