101 



have the same mode of articulation which restricts their bending in any other 

 direction but downwards, as in the Megatherioids generally. 



In the Ai, or Bradypus tridactylus, the metacarpal bone of the thumb is an- 

 chylosed at its lateral joint to the side of the base of the index, but retains its 

 articulation with the scapho-trapezial bone. This characteristic bone is less 

 concave towards the palm than in the Unau, and resembles more that bone in 

 the Mylodon. The lunare is relatively smaller than in the Unau or Mylodon, 

 and the part which penetrates the carpus is more wedge-shaped ; the subcubical 

 cuneiforme offers its flat surface more obliquely to the ulna, which here is more 

 prolonged upon the carpus, and the pisiform intervenes between it and the rudi- 

 ment of the fifth metacarpal. The chief modifications of both hand and foot in the 

 three-toed Sloth are the extensive anchyloses of different bones : this character is 

 manifested in the carpus by a coalescence of the trapezoides with the os magnum : 

 the resulting bone supports the base of the second metacarpal, and a great part 

 of that of the middle metacarpal ; thus fulfilling the same relations to the meta- 

 carpus as do the separated bones in the Unau. The rest of the middle metacarpal, 

 and the base of the conjoined fourth and fifth metacarpals, with the exception of 

 a small portion of the latter, are supported by the unciforme*, here, in confor- 

 mity with the normal development of the fourth finger, of larger relative size 

 than in the Unau. The rudiment of the fifth finger appears as a process from 

 the outside of the base of the fourth metacarpal, but is less than the correspond- 

 ing process which represents the thumb. The proximal phalanges are anchylosed, 

 as Cuvier has shown, to the metacarpal bones in the three-toed Sloth ; the ter- 

 minal phalanges have the same amount of resemblance to those of the Megalonyx 

 and Mylodon, as in the two-toed Sloth. 



In proceeding to compare the fore-foot of the Mylodon with that of its extinct 

 congeners, it becomes necessary to recompose and redescribe the bones of the 

 hand of the two species in which alone they have hitherto been obtained in 



* This bone M. deBlainville (luc. cit. p. 24) prefers to regard as the os magnum ; but, admitting that 

 the styloid process is not detached in the Ai, he gives its right name to the os triquetrum, which is his 

 unciforme in the Unau ; and consequently he states that the unciforme is absent in the carpus of the 

 Ai. After I had recomposed the carpus of the Mylodon, I proceeded, with the new and valuable light 

 which it afforded, to study the analogies of the carpus in the Sloths, and before referring to the works 

 of Cuvier and M. De Blainville, I wrote down my conclusions, which are those in the text. They are 

 identical with those of Cuvier. 



