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adjoining surface of the fourth metacarpal. The carpal facet is relatively larger 

 than in the Mylodon, and its radial half is convex from above downwards, indi- 

 cating a corresponding modification in the unciform bone : on the ulnar side of 

 the base is a rough, flattened protuberance, as in the Mylodon, without any 

 trace of an articular surface. The cast of the bone here described will not adapt 

 itself to the middle metacarpal ; and if, as Cuvier supposed, it even did not 

 belong to the same hand, yet if it were a fourth metacarpal, it ought, from the 

 analogy of both Megatherium and Mylodon, to have had a smooth, broad articular 

 surface, where the rough flat protuberance actually exists. The distal end of the 

 metacarpal in question presents the same modifications as in the fifth metacarpal 

 of the Mylodon : a simple vertically-oblong surface presents an arthrodial in- 

 stead of a ginglymoid joint for the proximal phalanx, below which there is a 

 surface for a sesamoid bone. The phalangeal surface is more uniformly convex in 

 the Megalonyx, and the sesamoid surface is flatter. The upper surface of the bone 

 does not present the oblique ridge which the figures of the Madrid Megatherium, 

 and the bone itself in the Mylodon present, but all the essential characters 

 prove the metacarpal in question to be the fifth of the same hand as the middle 

 and index ones ; and whilst these are modified in accordance with the long and 

 powerful claws which terminated their digits, it is evident, from the simplification 

 of the distal articulation of the fifth metacarpal, that the finger which it sup- 

 ported was as mutilated and short as in the Mylodon and Megatherium. 



The examination of the bones of the fore-foot of the Megalonyx, aided by 

 the comparison of those of the Mylodon, has thus conducted to very different 

 conclusions from those to which Cuvier arrived, guided by the analogy of 

 the Cabassou Armadillo. The Megalonyx had at least two of the digits of the 

 fore-foot short, strong, and armed with long claws ; but these were the index 

 and medius ; the fifth digit was more slender, and its phalanges were probably 

 only two in number, and invested by a thickened and callous integument like a 

 hoof. Whether the fourth digit off'ered a corresponding modification, as in the 

 Mylodon, or whether, assuming the accuracy of the figures of the Madrid 

 Megatherium, it also was terminated by a claw, must be decided by the evidence 

 of further discoveries of the bones of the Megalonyx. A pollex unquestionably 

 existed, and it is most probable that it was not rudimental, as in the Megathe- 

 rium, but completely developed as in the Mylodon, and that the small ungual 

 phalanx (fig. 9, in Cuvier's plate, above quoted) belonged to it. 



