140 THE MAMMALIA OF THE DEEP RIVER BEDS. 



both lower and narrower, but its upper surface rises rapidly towards the palmar side, 

 forming the " head." Behind the anterior face the bone is deeply constricted by two 

 concave facets, one on the radial side for the trapezoid and the other on the ulnar 

 side for the lunar. The trapezoid and magnum are very closely interlocked and form 

 a continuous saddle-shaped surface foi* the scaphoid, which in appearance resembles 

 the astragalar trochlea of a carnivore. The magnum, including even the head, is 

 entirely beneath the scaphoid, the opposite condition to that of the horse, in which 

 the head is entirely underneath the lunar, though the scaphoid rests upon the anterior 

 portion. The lunar surface is deeply concave (though less so than in Merycliyus and 

 Merycochce.rus, in which it describes a semicircle) and almost entirely lateral in posi- 

 tion, but on the palmar side is a small, shelf-like projection which extends somewhat 

 beneath the lunar. This gradual displacement of the magnum towards the radial 

 side of the carpus is already indicated in the oldest known genus of the family, Pro- 

 toreodon, and is more decidedly marked in Oreodon and JSporeodon, though even in 

 the latter it has by no means been carried to the same extent as in Mesoreodon, in 

 which it attains almost the extreme position found in Merycliyus and Merycoclicerus. 

 The contact between the magnum and the unciform is very slight and nearly or quite 

 limited to the posterior or palmar margin, the two bones being separated by the long 

 beak of the lunar and the strong process which the third metacarpal sends obliquely 

 upward and outward to abut against the unciform. Distally, the magnum bears a 

 single saddle-shaped facet for mc. iii, which is reflected upward more upon the ulnar 

 than on the radial side. This facet is elongate and quite deeply concave in the dorso- 

 palmar direction, contracting to a point behind, narrow and very convex transversely. 

 There is no facet for mc. ii, that bone being excluded from contact with the magnum 

 by the connection of mc. iii with the trapezoid. The posterior hook of the magnum 

 is short, curved, blunt, depressed and curved towards the radial side. The unciform 

 is high and broad, with its proximal portion contracting posteriorly. The upper sur- 

 face bears an oblique facet for the lunar, which rests almost entirely upon the unci- 

 form, and somewhat larger convex facet for the cuneiform. The metacarpal surfaces 

 form a nearly continuous curve. On the radial side, though confined to the dorsal 

 half of the bone, is a large oblique facet for the projection from mc. iii ; distally, 

 there is a larger facet for mc. iv and a smaller one for me. v ; the latter surface is 

 reflected up upon the ulnar side of the unciform. 



The metacarpals are four in number and in their proportions very similar to those 

 of the older genera, Oreodon and Eporeodon, though differing in some important 

 respects from the metacarpals of those genera in their mode of articulation with the 

 carpus, which is like that of Merycoclicerus and Merychyus in being of the " adaptive " 



