108 



galonyx than in either the Megatherium or Mylodon. The articulation on the 

 radial side of the base is nearly flat, hut is confined, as is the corresponding 

 convex protuberance in the Mylodon, to the upper half of that side of the base. 

 The flatness of the preceding surface governs a corresponding modification in 

 the articular surface of the adjoining metacarpal. 



This metacarpal * is proved by the analogy of the Mylodon not to belong, as 

 Cuvier supposed, to the annular or fourth, but to the index or second digit. It 

 is, however, relatively longer and narrower than in the Mylodon, in which the 

 concavity adapted to the tuberosity on the radial side of the third phalanx, oc- 

 cupies nearly the whole of the ulnar surface of the bone. The triangular base of 

 the second metacarpal in the Megalonyx is more inequilateral than in the My- 

 lodon : the emargination of the anterior border is deeper : the flat surface on the 

 radial side of the base, for the metacarpal of the pollex, very closely resembles 

 that in the Mylodon, whilst in the Megatherium it is concave. We may infer 

 from this that the entire pollex in the Megalonyx resembled that in the Mylodon, 

 instead of being reduced to a rudimental metacarpal bone as in the Megatherium. 

 The distal articulation of the second metacarpal corresponds pretty closely with 

 that in the Mylodon, the projecting ridge having a more convex contour than in 

 the third metacarpal ; the second finger, therefore, of the Megalonyx must have 

 had a greater extent of vertical motion than the third. The surface on the inner 

 or ulnar side of the base of the middle metacarpal, which Cuvier thought to be 

 the outer side, and to indicate the existence of a considerable metacarpal of the 

 index, gives of course the same indication in regard to the annular or fourth digit, 

 to which in truth it was adapted. 



From the analogies of both Megatherium and Mylodon, the fourth metacarpal 

 of the Megalonyx may be concluded to have been longer and more slender than 

 the middle metacarpal ; and with the greater probability, because its articulation 

 with that metacarpal was of relatively less extent than in the Mylodon or Mega- 

 therium. The metacarpal bonef , which Cuvier supposed to belong to the index 

 finger, corresponds with the fifth metacarpal in the Mylodon, but like that in the 

 Megatherium, it is relatively longer and more slender, and its shaft is smoother 

 and more cyUndrical. The proximal expansion presents two articular surfaces, 

 a terminal one for the os unciforme, and a lateral one on the radial side for the 

 » Cuvier, Ossem. Fossiles, Ed. cil., pi. 216, fig. 8. t Ibid. fig. 11. 



