56 



most part slightly convex : the surface on what may be termed the truncate end of the 

 production, for articulation with the unciforme, is slightly convex on the dorsal half, 

 slightly concave on the palmar half. The surface for the magnum on the proper base of 

 the metacarpal sinks into a conical cavity near its dorsal end ; the rest being nearly fiat. 

 On the radial side of the base is an oblong articular tract for the second metacarpal, 

 and beyond this the extensive rough surface for the ligamentous connexion with the 

 same bone. The radial surface is grooved above the rough facet, obliquely by a wide and 

 moderately deep canal ; apparently for the passage of a tendon to the digit. The 

 dorsal surface shows an oblique broad tuberosity, extending from above the tendinal 

 groove to the upper or dorsal end of the distal articulation. The smooth and almost 

 flat dorsal surface gradually deepens into a broad and shallow oblique channel on the 

 ulnar side of the oblique tuberosity. The ulnar side of the bone, beyond the articular 

 surface for the fourth metacarpal, is occupied by a rugged flat tract for ligamentous 

 connexion with the same bone. The palmar surface is pretty smooth, flat transversely, 

 slightly concave lengthwise ; produced into a tubercle below the middle prominence 

 of the distal joint. The articular surface of this joint does not cover the whole distal 

 end of the bone ; it is long and rather narrow, extending obliquely from the palmar 

 forward to the dorsal surface ; the dorsal side of the bone being longer than the palmar 

 one. It is traversed lengthwise by a median prominence, convex transversely, almost 

 straight lengthwise ; and the surface is continued upon a flat tract on each side of the 

 prominence, that on the radial side of the prominence being the broadest. 



The general form of this bone is more like that of a brick or of an ashlar stone for a 

 strong wall, than like that of the usual support of a flexible digit of a fore-paw or hand. 



The chief difference between the middle metacarpals of the Megatherium and Mylo- 

 don is in the form of the distal articulation. This surface, in the smaller Megatherioid, 

 is convex from above downward, whilst in the Megatherium it is straight, or rather 

 concave, and it joins the dorsal surface at an acute angle. The lateral depressions of 

 the pulley are narrower in the Megatherium, and the vertical inflexions of the phalanx 

 must have been more limited than in the Mylodon. 



The fourth metacarpal (ib. miv), as compared with the third, is longer and more 

 slender in the Megatherium than in the Mylodori ; but its articulation by an obliquely 

 extended base with the third and fifth metacarpals and the unciform bone, closely 

 corresponds with that in the Mylodon *. 



The two oblique metacarpal surfaces are nearly parallel, the radial one is concave, the 

 ulnar one slightly convex ; both are separated by a sharp angle from the intermediate or 

 carpal surface, which is nearly square and is slightly concave. The proximal half of the 

 bone is bounded by four flat equal sides, with intervening angles, sharp on the radial 

 side. The upper and under sides are nearly smooth, with a broad low tuberosity near 

 the proximal end ; the outer and inner sides are rugged for close syndesmosis with the 

 adjoining metacarpals. Only the distal half of the bone stands freely out ; it expands 

 • Description of the Mylodon robustut, 4to, p. 92. pi. xv. m 4. 



