51 



in thickness : the plate from this ridge sweeps round the back part of the bone, subsiding 

 and increasing in breadth as it approaches the radial side, where it terminates in a 

 tuberosity, divided by a groove from the radial articular cavity (ib. b), and prolonged 

 downward into the ridge bounding the radial side of the upper half of the ulna. The 

 olecranon projects rather backward than upward, and is strengthened and supported at 

 its highest part by a strong angular buttress, which gradually subsides upon the back of 

 the ulna. 



The great ' sigmoid' articular surface is divided into two facets by a median portion, 

 which is produced forward in the longitudinal or vertical direction, and is convex from 

 side to side ; the divisions so denned are concave. The inner one (ib. c), for the inner 

 humeral condyle, describes a semicircle from behind forwards ; it has little more than 

 half that extent from side to side, and is encroached upon by a narrow, rather deep, 

 rough channel, continued from the inner origin of the olecranon, expanding, to near 

 the middle of the cavity. The outer division of the sigmoid fossa (ib. b) has reverse 

 proportions, the transverse being nearly double the extent of the longitudinal diameter, 

 and it is less concave than the inner division ; about half an inch of its lower border 

 bends a little back for the articular margin of the head of the radius, just above the 

 rough fossa, for the reception of the non-articular part of the same head : the rest of the 

 outer division of the sigmoid cavity receives the back part of the outer condyle of the 

 humerus. Below the outer division the radial fossa (ib. d) presents a triangular form, 

 bounded by a rough ridge externally and by a tuberosity internally. The ridge is con- 

 tinued upon the radial side of the shaft of the ulna to within a fourth part of its distal 

 end ; the surface of this side of the bone is irregularly sculptured, indicating the strong 

 ligamentous connexion between the ulna and radius at this part. The back and inner 

 sides of the shaft of the ulna are comparatively smooth. 



A vascular canal, somewhat larger than the rest, is seen near the fore-part of the outer 

 surface, entering the bone obliquely upward. A rising of the surface, with a linear 

 series of three or four rough tubercles, marks the lower fourth of the inner side of the 

 bone ; a short wide longitudinal channel marks the back surface of the distal end 

 (fig. 3), which is slightly expanded and convex, and so impressed as to indicate its 

 ligamentous junction with the carpus. 



The ulna of the Megatherium differs from that of the Mylodon, not only in its longer 

 and more slender proportions, but also in the absolutely as well as relatively minor 

 height or length of the olecranon ; in the much less relative vertical or longitudinal 

 extent of the outer division of the ' sigmoid' cavity ; and in the ' haversian' fossa on the 

 inner division. It differs in the much narrower channel dividing the articular cavity 

 from that part of the base of the olecranon which is continued into the posterior ridge 

 or border of the shaft ; it differs, also, in the convexity of the distal end and the absence 

 of the articular facet, which is distinctly present in that part of the ulna in the Mylo- 

 don and Scelidotherium. 



Radius. — The radius (Plate XX. figs. 4, 5, 6), like the ulna, of the Megatherium 



