no 



The remains of the Scelidotherium collected by Mr. Darwin offer the follow- 

 ing bones of the hand for comparison with those of the Mylodon and Megathe- 

 rium, viz. the OS lunare, the os cuneiforme, and one of the ungual phalanges, 

 probably that of the large middle toe. The lunare is intermediate in its charac- 

 ters between those of the Megatherium and Mylodon : it is chiefly distinguished 

 by the very slight angle formed by the surfaces for the os magnum and cunei- 

 forme : the radial articular surface is less contracted towards the palmar aspect 

 of the wrist than in the Mylodon. The cuneiforme and lunare are separated by 

 a deeper and wider cleft than in the Mylodon, and the tuberosity which projects 

 from the cuneiform bone into that interspace is more developed. The surface 

 adapted to the truncated extremity of the ulna is flatter, and relatively larger than 

 in the Megatherium ; and it resembles that of the Mylodon in being continuous 

 with a slightly convex oval surface at the back part of the os cuneiforme, placed 

 at right angles to the ulnar surface, for the articulation of the os pisiforme. The 

 rough dorsal surface of the cuneiforme is neither so extensive nor so flat as in 

 the Mylodon, but rather resembles that surface in the Megatherium. The ungual 

 phalanx has the general proportions of the large one of the Mylodon, and is 

 consequently less compressed than that of the Megalonyx : it is nearly equal in 

 size to that of the middle digit in both. The chief distinctive character of this 

 phalanx in the Scelidothere is the flatness of the basal plate, and the development 

 from its middle line of a ridge-like process : the margins of the plate appear as 

 if obliquely bevelled ofi", or bent upwards to form the osseous sheath. The ulnar 

 cavity of the trochlea is deeper, as in the Mylodon, than the radial one ; the 

 ridge dividing them is narrower and better marked : the upper part of the claw- 

 process is rather sharper than in the Mylodon, but it is indented near the apex 

 as in that species and the Ant-eaters. Like the ungual phalanx of the Mylodon, 

 this of the Scelidothere illustrates in every particular the argument by which 

 Cuvier established the ordinal aSinities of the allied extinct genus Megalonyx. 



Thus whilst the resemblance that can be traced between the bones of the 

 fore-foot in the four genera of Megatherioid animals, above adduced, is sufiicient 

 to establish the unity of family-type on which that member was constructed, the 

 recognizable varieties concur with those presented by other bones, and by the 

 dental system, in establishing so many generic modifications of that peculiar 

 type. 



