106 THE MAMMALIA OF THE DEEP RIVER BEDS. 



extent and the two in front are widely separated; the superior one is narrower hnt 

 extends much farther back upon the side of the head. On the distal surface several 

 notable differences are to be observed. In the fossil, the anterior border describes a 

 decidedly smaller arc of a circle and the middle line of the posterior prolongation is 

 nearly coincident with the middle line of the bone, while in the horse the radial por- 

 tion of the magnum has been much extended, which gives to the hinder prolongation 

 a more unsymmetrical position ; it is also very much broader and its hinder margin 

 straighter and more oblique in direction, and forming angles with the lateral borders, 

 instead of being connected with them by curves as in A. equinum. The magnum of 

 A. aurelianense is altogether like that of the American species, except for the con- 

 fluence of the two anterior unciform facets. 



The unciform has, unfortunately, been lost from the manus of both sides, but 

 there is no reason to doubt its resemblance to that of the European species in all 

 important respects. 



The Metacarpus. The second metacarpal has a head of more primitive form 

 than in Equus ; it is less extended antero-posteriorly in proportion to its breadth, and 

 not only rises above the head of mc. iii, but slightly overlaps it, in order to reach the 

 magnum. The two facets for mc. iii are distinctly, though not so widel} r , separated 

 and the anterior facet is plane, not concave. As the head of mc. iii has a much 

 smaller extension towards the radial side, mc. ii is less crowded backward and is 

 more completely visible from the front. The surface for the trapezoid is less flat- 

 tened, but remains slightly concave, and passes on the palmar side into a small facet 

 for the trapezium, which is lost in the modern genus. The shaft is long, very much 

 compressed lateral^, but retaining a considerable antero-posterior depth ; the same 

 is true of the distal end, which measures almost as much from before backward as 

 does the distal trochlea of the median metacarpal. The carina is but feebly devel- 

 oped and entirely confined to the palmar side of the trochlea. In the specimens of 

 A. aurelianense from Sansan, which Kowalevsky has figured, mc. ii differs from that 

 of A. equinum only in the following particulars: The proximal portion is triangular, 

 not irregularly quadrate in shape ; the facet for mc. iii is not divided into two parts ; 

 and the distal trochlea is more recurved. Fraas has, however, figured a specimen 

 from Steinheim (No. 16, PI. VI, Fig. 12) in which the lateral metacarpals are 

 of proportionately enormous dorso-palmar extent, far more so than in the American 

 form. 



The third metacarpal differs in many important respects from that of Equus ; 

 the proximal end is much less expanded transversely, while its depth from before 

 backward remains relatively less. Owing to this less expansion transversely, the 



