103 



and is of an oval shape ; convex at the anterior and concave at the posterior 

 part* ; it is separated by a relatively wider space from the radial surface of the 

 lunare than in the Mylodon. The dorsal rough surface of the bone is more con- 

 vex, and has a smaller but better marked mamrailloid protuberance at the upper 

 and radial angle. The trapezoides is a relatively smaller and flatter bone than in 

 the Mylodon : the proximal or scaphoidal surface is convex transversely, concave 

 from behind forward, and plays in a corresponding concavo-convex surface in 

 the scaphoides. The distal surface is principally convex : both surfaces are joined 

 by a small articular facet on the radial side of the bone, which is adapted to a 

 corresponding facet in the small metacarpal bone of the thumb ; and by a more 

 extended articular surface on the ulnar side of the bone for junction with the 

 OS magnum. This carpal bone has a close correspondence of form with that of 

 the Mylodon, and has the same connexions in the carpus : its articular surface 

 for the trapezoides is of course much narrower, whilst the adjoining one for the 

 scaphoides is broader : the outer facet for the unciforme is narrower, and the 

 two anterior surfaces which enter the base of the great middle metacarpal meet 

 at a less open angle than in the xMylodon : the dorsal rough flattened surface of 

 the bone has thus a more regular transversely extended hexagonal figure, the 

 outer and inner sides being the shortest. 



The OS unciforme of the Megatherium differs most in form from that of the My- 

 lodon : it is a transversely elongated hexahedron, the outer side formed by a rough 

 projection, separating the surfaces for the os cuneiforme and fifth metacarpal, which 

 meet at an acute angle in the Mylodon. The opposite side, which articulates 

 with the lunare and os magnum, is rounded off. The three distal surfaces for 

 the three outer metacarpals are nearly in the same transverse line, which runs 

 parallel with that applied to the os cuneiforme. The dorsal surface is traversed at 

 its outer half by a transverse ridge analogous to that marked g in the figure of the 

 Mylodon's hand, Plate XV. ; but this ridge is continued in the Megatherium to 

 the middle of the outer surface of the bone, where it forms a projection between 

 two deep grooves ; the inner surface of the unciforme is characterized by a 

 median tuberosity as in the Mylodon. 



The metacarpal bone of the thumb of the Megatherium resolves one of the 



* In the Mylodon the pisiform bone is adapted to the flat surface of the cuneiforme, corresponding 

 to this concavity. 



