30 MEGALONYX. 



just referred to, and the oblique margin of the olecranon above the brachial articu- 

 lation is narrower. The distal end is narrower and thicker than in the specimen 

 of the Jeiferson collection, so that its styloid process appears less prominent ante- 

 riorly. 



Measurements derived from the fragments of the ulna are as follows : — 



Inches. Lines. 

 Greatest breadth of the ulna from the summit of the coronoid process . . 5J 

 Greatest breadth of distal extremity . . . . . . . .2 10 



Breadth at middle of shaft .......... 3 4 



Carpus. — As in Mylodon, the carpus of Megalonyx is composed of seven bones 

 placed in two rows, the odd bone, which is the homologue of the scaphoides and 

 trapezium being common to both genera. 



Of the seven bones, specimens of all are preserved in the collections of Drs. 

 Owen and Dickeson except one : the trapezoides. Dr. Owen's collection contains 

 the scapho-trapezium of both sides, a lunar, and a pisiform bone. Dr. Dickeson's 

 collection contains a lunar, a cuneiform, and a pisiform bone, and an os magnum. 



Os Scapho-trapezium. — (PI. VIII, Figs. 6, 7, 8, A.) The scapho-trapezial bone is 

 the largest of the carpus, and is irregularly pyramidal with the apex, formed by 

 the trapezial portion, curved forward. 



The proximal articulation (Figs. 6, 8, A) for the radius is one uniform convexity, 

 with an irregularly crescentic outline, extending the whole breadth of the bone. 



The distal surface (Fig. 7, A) is divided into three nearly equal portions ; one 

 on the outer side (c), an oblong convex articulation for the metacarpal of the 

 thumb; another towards the dorsum (cZ), deeply concave, for junction with the 

 trapezoides ; and the third is an irregular surface for ligamentous attachment. 



The ulnar side of the bone presents a reniform, articular surface, for the lunare, 

 and below this a rough, interarticular surface continuous with a deep gutter passing 

 to the distal side, and separating two articular facets for the magnum (Fig. 7, e,f) ; 

 that anterior being circular, that posterior, quadrilateral. 



The dorsal surface is inconspicuous, but the palmar surface (Fig. 6, A), is large 

 and concave. 



Os Lunare. — (PL VIII, Figs. 6, 7, 8, B; 9, 11, A.) The lunare is a wedge-shaped 

 bone almost a third smaller than the scapho-trapezium. Its proximal and distal 

 articular surfaces are separated by an irregular interarticular tract perforated by 

 vascular canals. 



The proximal articulation (Fig; 8, B; 11, A) is a broad, antero-posterior con- 

 vexity for conjunction with the radius, and is continuous on its outer side, at an 

 acute angle with a reniform articular surface corresponding to that of the scapho- 

 trapezial bone. 



Distally, the lunare presents a narrow antero-posterior concavity (Fig. 7, B, g), 

 for junction with the magnum; and it is continuous on the ulnar side nearly at a 

 right angle with a larger concave articular facet for the cuneiforme {h). 



As in the preceding bone, the palmar (Fig. 6, h) is more extensive than the 

 dorsal surface. 



