28 MEGALONYX. 



appears as if its margins had been pressed towards each other anteriorly so as to 

 form between them a concave gutter expanding below. The anterior surface of the 

 bone generally is slightly concave transversely, and is a little roughened for muscular 

 attachment. The posterior surface presents an oblique ridge extending along the 

 middle half, from which it slopes on each side to its margins. The outer margin 

 is obtuse, and in the specimen under examination presents the appearance of 

 having been gnawed by some rodent animal. The inner margin is thin at the 

 middle, but expands below into a broad, trilateral, rugged surface, for ligamentous 

 conjunction with the ulna; and at the upper part it is directed forward to the 

 bicipital tuberosity, which is convex and almost two inches long. 



The head of the radius is oval; and it measures two inches and two-thirds 

 antero-posteriorly, and two inches and a sixth transversely. Its brachial articular 

 surface is a lenticular concavity with the border convex ; and its ulnar facet is 

 demi-circular in outline and transversely convex. 



The carpal articulation (PI. IX, Fig. 6) is a deep trilateral concavity which is 

 open at the ulnar margin. It is three and a quarter inches broad, by two inches 

 and a third internally. Its outer end is bounded by a strong styloid process ; its 

 anterior margin is acute ; and posteriorly it is strengthened by means of a broad, 

 convex, and rough tuberosity. Between the latter and a ridge existing upon the 

 back of the styloid process there is a broad groove to accommodate the extensor 

 tendons ; and a second and deeper groove exists between the posterior part of the 

 styloid process and an oblique ridge ascending from the outer margin of the bone. 



Inches, lines. 



Length of radius to the end of the styloid process It 9 



Greatest breadth at the distal end 3 '7 



Breadth at the middle of the shaft 3 3 



The articular extremities of both radii contained in Dr. Owen's collection, corre- 

 spond pretty closely with the same parts of the specimen above described. The 

 brachial articulation is rather more circular, and in a trifling degree deeper ; and 

 the ulnar articulation of the head is a little smaller. The bicipital tuberosity is 

 stouter, but is directed less forward, so that the contiguous anterior surface of the 

 bone appears less concave ; inferiorly, however, the continuation of the same sur- 

 face is more concave transversely. 



The measurements of the fragments are as follows : — 



Inches. Lines. 



Antero-posterior diameter of the head . 2 8 



Transverse diameter of the head . 2 2 



Breadth of distal extremity 43 



Breadth of carpal articulation 83 



Greatest antero-posterior diameter of the carpal articulation . . . .24 



Ulna. — (PI. IX, Fig. 5; X, Fig. 1.) The specimen of an ulna in the collection of 

 Jefferson is a broad, compressed bone, which, like the radius, is slightly bent ante- 

 riorly. Its dorsal aspect and inner margin are longitudinally convex, while the 

 palmar aspect and radial margin in the same direction are concave. 



The dorsal and palmar surfaces of the shaft are slightly depressed, and are 



