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smallest facet for the small part of the base of the fifth metacarpal which articulates 

 with the carpus. The inner or palmar rough surface of the unciforme has an oblong 

 tuberosity, which is narrower than that upon the dorsal surface. 



Metacarpus. — The innermost or first metacarpal (Plate XXI., I, m), answering to 

 that of the pollex or thumb, resolves, by its rough obtuse distal termination, as well as 

 by its diminutive size, one of the points considered doubtful by Cuviek, in the structure 

 of the fore-foot of the Megatherium, by proving that the pollex was absent, or repre- 

 sented solely by the rudimental metacarpal, which must have been concealed beneath 

 the integument. The bone is of an irregular subcubical shape, broader than it is long. 

 At its base are two separate subcircular flat surfaces articulating with corresponding 

 facets on the trapezial part of the scapho-trapezium : on the side next the second meta- 

 carpal is a convex articular surface, having the lower part obscurely defined for articu- 

 lation with the trapezoides, and the rest lodged in the concavity, partly articular, partly 

 rough, upon the outside of the base of the second metacarpal. The outer side of the 

 rudimental first metacarpal is obliquely flattened, the surface here being apparently for 

 the insertion of a strong tendon. 



The metacarpal of the second digit (ib. m n *) has a very irregular exterior ; its 

 comparatively small base is excavated obliquely to receive the fore-part of the trape- 

 zoides ; on the outer or radial side it shows a triangular excavation, the lower half of 

 which has a smooth articular surface for the rudimental metacarpal, m I. On the 

 ulnar side of the base there is a convex articular surface divided into a proximal narrow 

 tract for the os magnum, and a distal broader tract for the contiguous side of the base of 

 the middle metacarpal. Above or beyond this the middle third of the bone supports a 

 rugged protuberance, which has been attached by ligament to a similar rough surface on 

 the middle metacarpal ; so that little more than one-third of the bone projects freely 

 from the metacarpus. This free part is subcompressed and expands in the direction 

 from the back to the palm of the hand, so as to form the surface for a trochlear articu- 

 lation of great extent in that sense. A ridge from the back surface of the metacarpal 

 expands into a tuberosity near the dorsal end of that articulation ; and a similar 

 but smaller tuberosity projects from the palmar end of the same articulation, along each 

 side of which there runs a thickish edge. The distal surface itself presents a median 

 vertical or longitudinal prominence, beyond the lower half of which the articular surface 

 is produced laterally so as to make the surface concave transversely on each side the 

 median ridge ; whilst at the upper half the whole surface is convex transversely, as it is, 

 in a minor degree, longitudinally. 



The middle metacarpal (ib. m in) is a little longer than the second, but is twice as 

 thick, with a quadrate transverse section, the four sides of the shaft being flat and sharply 

 defined. The base of the bone is produced ' ulnad' and ' proximad,' so as to be wedged 

 between the fourth metacarpal, the magnum and the unciforme. The articular surface 

 on the ulnar side of this production for the fourth metacarpal is extensive, and for the 

 * Kel'erred to the 'annulaire' or fourth digit in the ' Ossemens Fossiles,' ed. cit. pi. 216, fig. 8. 



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