57 



in vertical breadth to the articular surface for the fourth finger. The angle between 

 the upper and radial sides is rounded off; that between the upper and ulnar sides is, in 

 one specimen, developed into a sharp ridge. 



The distal articular surface resembles that of the second metacarpal, but occupies a 

 smaller relative proportion of the whole distal surface ; the vertical prominence, convex 

 transversely, is broader, more concave vertically, and extends obliquely from the upper 

 and radial angle to the lower and ulnar one : the flat lateral extension of the articular 

 surface is confined to the radial side of the prominence. A single flat surface for a sesa- 

 moid bone is situated below, but distinct from, this part of the articulation. A large 

 and strong tuberosity projects below the prominent part of the joint ; an oblique channel 

 divides this tuberosity from a longer one on the ulnar side of the distal end of the meta- 

 carpal. 



The distal articular surface of the fourth metacarpal in the Mylodon is reduced to a 

 small, vertical, oblong, nearly flat surface ; this difference relating to the stunted deve- 

 lopment and limited function of the digit it has to support. In the Megatherium such 

 simplification of the distal joint of the metacarpal is limited to the fifth of that series of 

 bones (ib. m v) : this metacarpal *, of the same length as the fourth, is more slender ; its 

 proximal end is wedge-shaped, the radial articular surface and the flattened outer facet 

 converging to the narrow rough tract which is joined by ligament to the carpus. The 

 articular surface on the radial side has a small terminal part obscurely marked off for a 

 facet on the unciforme ; the rest receives the convexity on the ulnar side of the fourth 

 metacarpal : beyond this is a rough surface for the syndesmotic union of the contiguous 

 bones. Rather more than half the fifth metacarpal stands freely out; it is traversed 

 above by a longitudinal ridge expanding into a broad tuberosity at the distal end ; the 

 bone is smooth and rounded on the palmar side. The small terminal articular surface 

 is oval, slightly concave vertically, concave transversely, upon the radial half of the distal 

 expansion. There is no articular surface for a sesamoid. 



The metacarpals are so united and wedged together and with the carpus, as to trans- 

 mit from the oblique carpal surface which sustains the radius the weight of the fore 

 part of the Megatherium to the fifth digit, the stunted extremity of which was imbedded 

 in the marginal hoof-like callosity on which the ponderous quadruped trod, with the 

 claw-bearing toes bent inwards. 



From the scapho-trapezium and lunare the weight was transmitted to the second row 

 of carpal bones ; and by the oblique production of the base of the second metacarpal, 

 and especially of the third metacarpal, it was concentrated through the medium of the 

 fourth metacarpal upon the fifth. The lateral pressure thus occasioned explains the 

 extent of syndesmotic and sutural union along the basal part of the metacarpals, and also 

 the squared angular shape of the constituents of this masonry of the huge fore-paw. 



On comparing the structure of the carpus in the Megatherium with that in existing 

 Mammals, it is found to be repeated in the Unau or Two-toed Sloth (Bradypus (Choice- 

 • Beferred to the index or second digit in the ' Ossemens Fossiles,' ed. eit. pi. 16. fig. 11. 



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