366 R. S. Lull — Pleistocene Ground Sloth, 



It will be seen that the measurements of the two bones bear 

 a pretty constant ratio to one another. The three measure- 

 ments wherein the greatest discrepancy is shown were taken 

 over the articular surfaces themselves, which would naturally 

 be relatively larger in an older and more powerful animal. 



Radius and ulna. — The radius is practically perfect though 

 somewhat abraded, and from its size and general appearance 

 has been assigned the same catalogue number as the humerus, 

 No. 10265. 



This bone is straight, with a well defined proximal dilatation 

 which bears the concave humeral facet. Of this the front 

 margin is not preserved, but the shape must have been oval, 

 with a somewhat flattened portion where it adjoins the ulnar 

 facet. Distally the radius expands rapidly to a diameter half 

 again as great as the proximal end and two and a quarter times 

 as great as the least lateral extent of the shaft. The shaft is 

 trihedral, the front, rear, and inner aspects being separated 

 from one another by rather sharp angles, while the inner 

 front angle bears a well defined tuberosity for the insertion 

 of the biceps muscle. This tuberosity lies about one third 

 of the distance from the proximal end of the bone, giving 

 a comparatively great leverage for wielding the forearm and 

 hand. The distal aspect presents two distinct facets, a con- 

 cave one for the articulation with the lunar and a somewhat 

 saddle-shaped one for the scaphoid bone. Of these the lunar 

 facet has the greater area and is sub-oval in shape, the lesser 

 scaphoidal facet is irregularly triangular, with a sharply convex 

 antero-posterior axis, while the transverse one is slightly 

 concave. Great mobility of wrist is indicated in keeping 

 with the freedom of rotation of the forearm, the whole 

 mechanism implying ample prehensile powers. I can only 

 compare the bone with that of Mylodon robustus, from which 

 it differs in the more constricted shaft just below the proximal 

 articulation and the lesser area of the humeral facet as com- 

 pared with the " convexity [ulnar facet] lodged in the small 

 sigmoid cavity of the ulna" (Owen). 



Distally the proportions of the bones seem to vary, that of 

 M. robustus being thicker for its width, especially toward the 



Dimensions. 



Length over all ._- 270 ,nm 



Proximal diameter, transverse . 69 



ant.-post. (est.) 45 



Least diameter of shaft, transverse . 42 



ant.-post 34 # 6 



Least girth of shaft » . . 13 



Distal diameter, transverse _ _ 100 



ant.-post 64 



