62 



resembles in its longer and more slender proportions that bone in the Megalonyx, and 

 differs from the proportionally thicker and shorter radius of both the Mylodon and 

 Scelidotherium. The proximal end is circular, and is occupied by a smooth, moderately 

 shallow, articular cavity (ib. fig. 5), with a well-defined border, over which the articular 

 surface extends, on the ulnar side of the head, for about half an inch down ; which tract 

 is adapted to the lower portion of the outer division of the sigmoid cavity of the ulna. 

 The articular modification of the head of the radius is as completely adapted for the 

 superadded rotatory movements of the antibrachial bones, as in the human subject, to 

 the head of the radius of which the resemblance of that of the Megatherium is strikingly 

 close. 



The shaft of the radius gradually narrows, in the antero-posterior diameter, along the 

 upper fourth part, but maintains the same diameter as the head, transversely. Three 

 inches below the head, on the inner and fore-part of the shaft, is the tuberosity (ib. a) 

 for the tendon of the biceps, which measures 3 inches in long diameter and If inch 

 across. Here the bone bends a little outward (radiad) ; and the ridge bounding that 

 side is developed into what may be termed a process (ib. b), with a low angle, whence the 

 ridge is continued straight down the lower half of the shaft to near the tuberosity above 

 the styloid process (ib. c), where it curves outwardly to terminate in that tuberosity. 

 The fore-part of the shaft is moderately smooth and convex across ; it describes, length- 

 wise, a slight concavity ; on the inner side of the bone, a broad and very rugged tract 

 begins, about an inch and a half below the bicipital tuberosity, and extends along the 

 middle third of the shaft ; a less rough tract is continued thence, gradually expanding 

 to the cavity for the lower end of the ulna. The outer side of the shaft of the radius is 

 smooth, convex across, and with a slight convexity in its longitudinal contour : from the 

 external process downward the radius maintains an equal breadth near the lower end, and 

 there it expands in all directions to form the large articular cavity (ib. d and fig. 6) 

 for the major part of the carpus. Above this cavity, on the ulnar side, is the rough 

 and shallow depression for the ligamentous junction of the corresponding end of the 

 ulna; on the front side a broad low ridge extends obliquely from the suprastyloid 

 tuberosity to the border of the cavity ; on the back part three oblong, short, thick ridges 

 or tubercles divide the surface into four channels, for tendons ; three of these are longi- 

 tudinal and parallel, progressively increasing in width from the innermost (or one next 

 the ulna) to the third ; the outermost passes obliquely between the suprastyloid tubercle 

 and the styloid process. This process is short and thick, rounded at the end ; flattened 

 in front, with the smooth articular surface continued for a few lines upon the lower 

 border of this surface, from the general articular cavity which is extended over the lower 

 end of the radius including the styloid process. This cavity presents a triangular form 

 with the angles rounded off; the base next the ulna is short and oblique ; the anterior 

 border or side is the longest, the opposite border is the thickest, and is notched near the 

 styloid process : the cavity is moderately concave, and articulates with the scaphoid and 

 lunar bones of the carpus. 



