32 MEGALONYX. 



Os Unciforme. — (PI. VIII, Figs. 10, 12, 13, B.) The unciform bone is irregu- 

 larly pentahedral, and is the third in size of the carjjus. 



Proximally it presents a large, transverse, oblong square, sigmoid facet (Figs. 10, 

 12, e), for articulation with the cuneiform bone, and this is continuous on the outer 

 side with another and smaller facet for the os magnum. The distal articular sur- 

 face (Fig. 13, B) is tripartite, and irregularly trapezoidal in outline, and it joins the 

 median metacarpal (e), the annular metacarpal (/), and the auricular metacarpal 

 (g) bones. 



The dorsal and palmar (Fig. 10, B) surfaces are large, the former contributing 

 more to the back surface of the wrist than in any of the other carpal bones. 



Of the Carpal Bones in Conjunction. — The proximal surface of the carpus of 

 Megalonyx, as formed by the scaphoid, lunar, and cuneiform bones, presents a broad 

 convexity interrupted between the two latter bones (PI. VIII, Figs. 9, 11) by a 

 deep gutter with irregular and perforated sides, which corresponds to the interval 

 between the radius and ulna. The dorsal surface of the carpus is convex, and is 

 most contributed to by the cuneiform, unciform, and pisiform bones. The palmar 

 surface is concave, and is much deepened by the position of the pisiform bone. 



In the radio-ulno-carpal articulation the same synovial sac appears to have existed 

 between the radius and scapho-trapezium and lunare, and between the latter two 

 bones, and a second sac appears to have been situated between the ulna, cuneiform, 

 and pisiform bones. Between the two rows of carpal bones the synovial sac appears 

 to have been continuous and to have extended downwards between the bones of the 

 lower row, to be reflected throughout the carpo-metacarpal articulation. 



Metacarpals. — Of metacarpal bones the collection of Dr. Owen contains the left 

 index, both median, and the right annular ; that of Jefferson the left index, median, 

 and auricular; that of Col. Wailes, the left annular and auricular; and that of Dr. 

 Dickeson the right index. 



Of the metacarpals from that of the index finger to the last finger, the annular 

 is the longest and the index the shortest ; and the auricular, which is second in 

 length, is the narrowest, whilst the median is the most robust. 



The index metacarpal bone (PI. X, Figs. 15, 16, 17), of Dr. Owen's collection, 

 has an irregular cylindroid shaft, which, from the enlargement of the extremities, 

 is longitudinally concave. 



The proximal extremity (Fig. 16), is trilaterally prismoid, with the apex palmar 

 and tuberous, the inner angle prolonged, and the base notched. Upon its carpal 

 aspect it presents a nearly vertical, trilateral, sigmoid surface for articulating with 

 the trapezoides. Its inner side exhibits a fan-shaped, concave facet (Fig. 15), con- 

 tinuous at an acute angle with the carpal articular surface, for junction with the 

 median metacarpal ; and its outer side (Fig. 17) presents another facet, also con- 

 tinuous with the carpal surface, demi-pyriform in outline and slightly convex, for 

 articulating with the metacarpal of the thumb. 



The distal extremity (Fig. 15) is composed of a long, vertical, ellipsoidal, arti- 

 cular process, with a vertical convex offset upon each side, for junction with the 

 first phalanx. The outer articular offset is the larger, and above it there exists a 

 strong ridge mounting to the summit of the articular process for ligamentous 

 attachment. 



