59 



in which was implanted the flexor tendon, shows many linear impressions radiating from 

 the median tuberosity, forward and laterally. The articular surface excavates obliquely 

 the base of this phalanx which overhangs the joint ; the concave border of the median 

 angular prominence describes a semicircle from above forward and downward ; the ulnar 

 excavation is longer in that direction than the radial one. The well-marked trochlear 

 joint restricts the movements of the great claw to flexion and extension on a vertical 

 plane, whilst its position effectually prevents retraction of the claw, the preservation of 

 the effective condition of which is due to an opposite bend to that in the Cat-tribe, 

 as was well explained by Cuvier in his description of the Megalonyx*. The major 

 part of this enormous phalanx goes to form the sheath of bone protecting the base 

 of the claw. The bony core or peg on which the claw was fixed and moulded is 

 compressed, with a sharp edge above and flat below, where it projects beyond the 

 sheath. 



The finger of the fourth digit has three phalanges and is unguiculate, with a claw 

 resembling in shape and size that of the second digit. The proximal phalanx (ib. iv, i) 

 is so compressed in the direction of its axis, that the proximal and distal articular surfaces 

 almost meet above, only about 1^ line's breadth of rough surface there intervening : the 

 proximal articular surface is a wide and moderately deep vertically-elongated channel, 

 with a semielliptic flat surface continued from the lower half of the radial border ; the 

 outer and inner sides of the phalanx are narrow, elliptic rough convexities ; the distal 

 articular surface is gently convex vertically, sinuous laterally. The second phalanx 

 (ib. iv, i) is deeper than long ; its upper surface is produced backward over the proximal 

 phalanx into a rough obtuse process ; the under surface is much shorter, but is broader 

 than the upper surface ; the convexity of the distal articular surface describes a semi- 

 circle from above downward, and is divided by a vertical trochlear groove into two parts, 

 of which the ulnar one is the larger. The ungual phalanx (ib. iv, s) is long, triedral, 

 with the radial side of the sheath vertical, the under surface rough and flattened, and 

 the ulnar surface sloping from above downward and outward to join the under surface. 

 The upper posterior tuberosity, overhanging the joint, inclines radiad, and is somewhat 

 flattened ; the proximal surface presents a median vertical rising for the channel in the 

 preceding joint. 



The small elliptical surface on the radial half of the distal end of the fifth metacarpal 

 supports a very short lamelliform phalanx, produced outward some way beyond the 

 joint ; and to this is articulated a stunted second phalanx with a rough, obtuse, non- 

 articular end. In some instances the above two phalanges are blended together. 



Amongst existing Mammals the Sloths alone present the connation of the scaphoid 

 with the trapezium, the Two-toed Anteater and the Armadillo that of the first with the 

 second phalanx. This latter character, peculiar to the middle digit in the Megathe- 

 rium, is limited also to the same digit in the Myrmecophaga didactyla ; in Basypus it 

 affects the third, fourth and fifth digits. 



• Ossemens Fossilea, ed. cit. t. viii. p. 510. 



