100 



in the Mylodon. The base of the metacarpal bone of the thumb is expanded 

 transversely, and abuts by one extremity against the trapezium, and by the other, 

 at right angles to its axis, against the base of the second phalanx. The trape- 

 zoides is a small bone, articulated, as in the Mylodon, with the scapho-trapezial, 

 the OS magnum, and the second metacarpal : the os magnum presents almost 

 the same pentagonal contour as in the Mylodon, the anterior facet being also 

 convex, for adaptation to a median concavity in the base of the middle meta- 

 carpal, both sides of which are expanded, one to abut against the base ot the 

 second metacarpal, the other extending to the os unciforme, and interposing 

 itself between the os magnum and the fourth metacarpal. The atrophy of the 

 fourth and fifth fingers, which we observe to have commenced in the Mylodon 

 by the disappearance of the ungual phalanges, has proceeded in the Unau to the 

 removal of all the bones, save the metacarpal of the fourth finger, which is only 

 half the size of that which forms the vestige of the thumb on the radial side of 

 the hand ; it rests, as a great part of the corresponding metacarpal in the My- 

 lodon does, upon the expanded base of the third metacarpal. The os cunei- 

 forme presents a cuboid form., rather less regular than in the Mylodon, with a 

 flat surface for the truncated end of the ulna, but it extends further in the 

 direction of the ulna than it does in the ]Mylodon, and so has led M. de Blain- 

 ville * into the belief that it represents the styloid process of that bone per- 

 manently detached. The cuneiforme, however, presents a distinct articula- 

 tion for the subcircular surface of the depressed pisiform bone, which closely 

 resembles that in the Mylodon ; the connections of the cuneiforme with 

 the lunare and unciforme likewise correspond with those in the Mylodon, 

 but that with the little finger is of course wanting, since the finger itself does 

 not exist. 



The first or proximal phalanges are very short, and offer, as in the Mylodon, 

 a median channel on both articular extremities, one for the ridge on the distal 

 end of the metacarpal, the other for that on the proximal end of the second pha- 

 lanx ; the other phalanges present modifications of length adapted to their pre- 

 hensile ofiices ; but the ungual phalanges, except in the less development of the 

 osseous sheath of the claw, maintain a close correspondence of structure with 

 those of the Megalonyx ; they are more compressed than in the Mylodon, but 



* Osteographie des Paresseux, p. 13. 



