370 R. S. Lull — Pleistocene Ground Sloth, 



Eight metacarpal IV. — This is a splendidly preserved bone 

 of remarkable form, showing a strongly depressed shaft and 

 widely expanded articular ends. It is extremely rugose for 

 tendon attachment. Proximally there are three distinct though 

 continuous facets which bore respectively upon the unciform and 

 the two adjacent metacarpals. Of these the unciform and the 

 fifth metatarsal facets met in a high, straight ridge about 4r3 mm 

 long, which coincides with the anconal-palmar diameter of the 

 foot. Of these facets, that which bears against the fifth meta- 

 tarsal is very slightly concave and is toward the anconal side 

 of the foot, covering about half the expanded area of the prox- 

 imal end of the bone. The facet which articulates with the 

 unciform is much the larger and is first convex and then con- 

 cave in one dimension and straight in the other. The plane 

 of this facet forms an angle of about 55° with the long axis of the 

 bone and while the greater part lies on the inner aspect it 

 seems to include the straight ridge and about 6 nim of the outer- 

 surface of the metacarpal. The third proximal facet, that 

 for metacarpal III, is slightly convex in all diameters and 

 forms a truncated ellipse, the lines of demarcation with the 

 unciform facet causing a break in the curve. Along this line 

 the outline is slightly concave instead of convex. The angle 

 formed with the axis of the bone is now about 40°. Distally 

 the articular face is most peculiar, the keel being oblique 

 instead of normal to the plane of the foot, giving a torsional 

 appearance to the bone and implying that the digits instead of 

 lying prone upon the ground lay more or less on their outer 

 side with the claws turned inward. On the anconal side the 

 metacarpal is stiffened by a longitudinal buttress arising on 

 the distal expansion and running up the shaft, parallel with the 

 inner aspect, for about two-thirds the length of the bone. This 

 buttress divides the surface into two unequal concave areas. 

 The inner aspect is keeled, the keel running from the distal 

 dilatation to the facet of the third metatarsal, to which it lends 

 support. The palmar side of the bone bears a prominent 

 tubercle at the proximal end, some distance below which lies a 

 distinct smooth-walled channel running across the bone in a 

 slightly oblique direction. 



Dimensions. 



Length 108 mm 



Proximal width 46*5 



" antero-posterior diameter . 55 # 5 



Shaft, width 23 



antero-posterior diameter _. 35 "5 



Distal width _ _ _ _ 40 



" antero-posterior diameter 50 



