104 



doubtful points in the structure of the fore-foot of this animal, by proving that it 

 was the sole representative of that digit : the bone seems to be analogous to the 

 transversely extended base of the same bone in the Myloaon, and offers two 

 articular surfaces : one of these is adapted to the small flat oval surface, before- 

 described, on thetrapezial process of the scaphoid ; the other is a larger convex 

 surface, nearly at right angles to, and remote from the preceding, and divided 

 into three facets : the smallest of these is for the trapezoides ; the other two form 

 the convexity entering the outside of the base of the second metacarpal bone. 

 This bone is figured with tolerable accuracy in right proportions and relative 

 position in pi. 4. fig. 28 of Pander and D 'Alton's Treatise. 



The form and position of the middle metacarpal bone in the same figure cor- 

 respond with those of the specimen now before me, except that the four basal 

 facets are separated by sharp and well-defined angles, whilst they are too much 

 rounded in Pander's figure. Compared with the corresponding bone in the 

 Mylodon, this middle metacarpal of the Megatherium is longer in proportion to 

 its breadth ; especially along the dorsal surface, owing to the anterior production 

 of the dorsal half of the distal trochlear ridge, and the large protuberance above 

 that ridge. The middle facet of the basal articulation has no rough, non-articular 

 depression anteriorly, but is everywhere smooth and concave, though least so 

 towards the palm : the outer or radial facet, which is a convex protuberance in 

 the Mylodon, is here nearly flat, and extends from the dorsal to the palmar aspect 

 of the base : the ulnar facet is not separated by a rough depression from the 

 middle one, as in Mylodon, but meets it at a sharp angle : the distal surface 

 of the ulnar extension of the base of the middle metacarpal presents a moderate 

 and regular convexity, upon which the radial half of the base of the fourth meta- 

 carpal rests. The under or palmar surface of the middle metacarpal in the 

 Megatherium is flattened, and at right angles to the very rugged outer and inner 

 sides of the bone : the radial side is produced at its distal half into an oblong 

 protuberance ; but the general form of the bone is a four-sided short and strong 

 column. The most important difference between the middle metacarpals of the 

 Mylodon and Megatherium is in the form of the distal articulation, the median 

 ridge in the Megatherium being slightly concave in the vertical direction instead 

 of convex, and joining the rough dorsal surface of the bone at a right angle. 

 The lateral depressions of the pulley are relatively narrower, especially the ulnar 



