50 



Scelidothere. The articular head forms a larger proportion of a sphere, and projects 

 more freely beyond the tuberosities : these are relatively smaller, and are more equal 

 than in the Mylodon or Scelidothere: the external tuberosity, in particular, is more 

 developed in these smaller Megathcrioids. In them also the external ridge is continued 

 from above the ' musculo-spiral ' groove inwards, along the front of the humerus to the 

 apex of the deltoidal tract, forming its outer boundary : in the Megatherium, a smooth 

 concave surface divides the outer ridge from the deltoidal elevation, which is absolutely 

 narrower. The vertical outline of the back part of the shaft of the humerus in the 

 Megatherium is almost straight, being but a little bent forwards at its lower third, as it 

 is likewise in the Megalonyx ; and, in both, the olecranal depression is well defined : in 

 the Mylodon and Scelidotherium, the same outline of the humerus is slightly concave ; 

 the lower third of the bone being, as it were, a little bent back below the deltoidal 

 platform; and the olecranal fossa is not defined. The inner supracondyloid plate is 

 produced at its upper part into a strong tuberosity, in both the Mylodon and Scelido- 

 therium, but not in the Megatherium. 



In the existing Sloths, the humerus at this part, viz. above the inner condyle, is per- 

 forated in one genus (Cholcepus) and not in another (Ackeus, F. Ccjvier) : and the same 

 difference occurs in the great extinct Sloths. In the Megatherium the humerus is 

 imperforate, as it is in the Mylodon: in the Megalonyx and Scelidotherium it is per- 

 forated above the inner condyle. Yet the Megalonyx most resembles the Megathe- 

 rium, not only in the general proportions of the humerus, but in the configuration of 

 its two articular ends. The inner or ulnar condyle, e. g., is convex in every direction in 

 the Megalonyx as in the Megatherium : in the Mylodon and Scelidotherium it is convex 

 only from before backwards, but is concave from side to side. In the more robust pro- 

 portions, in the shape of the articulations and the development of the processes of the 

 humerus, the Mylodon and Scelidotherium as closely resemble each other, as the Mega- 

 therium resembles the Megalonyx in the same characters ; yet the humerus of the Sceli- 

 dotherium has the inner perforation, and that of the Mylodon a groove merely, for the 

 brachial artery and nerve. This variety, and the corresponding one above noticed 

 between the Megatherium and Megalonyx, show the inapplicability of the final cause 

 commonly assigned to the ento-condyloid hole, the existence or otherwise of which 

 depends merely on the ossification or non-ossification of the aponeurosis extending from 

 the shaft of the humerus to the ento-condyloid process. 



Ulna. — The bones of the fore-arm in the Megatherium, as in the Megalonyx, ex- 

 emplify by their greater length, as compared with those of the same segment in the 

 hind limb, the Bradypodal affinities of these huge extinct quadrupeds. 



The ulna in the Megatherium (Plate XX. figs. 1, 2, 3) is, however, peculiar for the 

 vast expanse of its proximal end, in connexion with its long and slender shaft. The 

 olecranon (ib. a) is twice as broad as it is long ; its inner border, springing from the notch 

 which penetrates the inner and back part of the humeral articular fossa, extends obliquely 

 upward and inward for 3£ inches, the ridge or edge of the plate being about an inch 



