105 



one, which hardly can be said to extend beyond the base of the ridge. A sur- 

 face for a sesamoid bone is extended upon the under part of the articulation 

 only on the right side, and the vertical inflections of the phalanx must have been 

 more limited than in the Mylodon. 



The fourth metacarpal bone, as compared with the third, is longer and more 

 slender in the Megatherium than in the Mylodon ; but its articulation by an ob- 

 liquely extended base, with the third and fifth metacarpals, and the unciform bone, 

 closely corresponds with that in the Mylodon : the characteristics of the fourth me- 

 tacarpal of the Megatherium are these ; the two oblique metacarpal surfaces are 

 nearly parallel, and are both separated by a sharp angle from the intermediate or 

 carpal articular surface, which is nearly square and slightly concave. The proximal 

 half of the bone is four-sided, the upper and lower sides nearly smooth and slightly 

 concave, the outer and inner side rugged for ligamentous attachments to the ad- 

 joining metacarpals : the distal half of the bone expands in vertical breadth ; the 

 angle between the upper and radial sides is rounded off, while that between the 

 upper and ulnar sides advances upon the upper surface, and is developed into a 

 sharper ridge. The distal articular surface is less simple than in the corresponding 

 metacarpal of the Mylodon, and is more like that of the middle phalanx ; it is not 

 merely confined to the broad convex vertical ridge, but extends upon the concavity 

 to the radial side of that ridge : a single flat surface for a small sesamoid bone is 

 situated below, but distinct from this part of the articulation. Unfortunately, 

 the proximal phalanx of the fourth toe is wanting ; and whether, therefore, this 

 toe actually terminates by a large ungual phalanx, as is represented in the figures 

 of the Madrid specimen ; or whether that phalanx be wanting, and the fourth toe 

 terminates as in the Mylodon, and like the fifth toe of the Megatherium itself, 

 still remains to be decided. 



The analogy of the Mylodon leaves scarcely any doubt as to the accuracy of 

 the condition of the fifth toe, as represented in the different figures of the Madrid 

 skeleton. This analogy confirms the opinion of M. Laurillard, who, in an 

 annotation to the text of Cuvier descriptive of the manus of the Megatherium, 

 dissents from the conjecture of the author as to the transposition of the fore-feet 

 in the Madrid skeleton. 



The valuable light yielded by the same analogy guides us still further in the 

 right interpretation of this remarkable and most important part of the organi- 

 zation of the Megatherium. In the essential character of the carpal organization, 



o 



