95 



apex, where it is slightly excavated longitudinally *. The under surface presents 

 a median convexity between two sUght concavities, separated by a sharp angle 

 from the sides of the claw-process. The rugged and perforated surface of all the 

 preceding ungual phalanges gives them the appearance of models in cork. 



The distal articular surface of the fourth and fifth metacarpals would of them- 

 selves have announced a considerable modification of the digits which they sup- 

 ported, so much diminished and simplified are these surfaces. The phalanges, 

 nevertheless, exhibit so considerable an abbreviation and simplification of form, 

 that only the exact adaptation of their articular surfaces could have produced 

 the conviction of their true nature. The proximal phalanx of the fourth digit is, 

 like the preceding ones, shorter than it is broad, and of greater vertical than 

 transverse extent. The proximal articulation is a long oval concavity, adapted 

 to the convex upper two-thirds of the distal articulation of the metacarpal ; the 

 lower third of that surface, on which alone a median ridge is developed, being 

 adapted to two large sesamoids, which articulate, also, with a small portion of 

 the under surface of the phalanx. The radial side of the phalanx is flat ; the 

 ulnar side forms a rough protuberance. The distal articulation is less than half 

 the size of the proximal one, and occupies only the upper half of the distal sur- 

 face of the phalanx : it is convex vertically, concave transversely : below it is a 

 rough and deep cavity separating two tuberosities. The second and last phalanx 

 is not longer, but is narrower, especially in the vertical direction, than the first ; 

 its proximal articulation is precisely adapted to the concavo-convex surface on 

 the first phalanx ; the upper surface of the bone forms a protuberance above 

 that surface : the lower and broader part is flattened, but has a central depres- 

 sion : the upper and distal surfaces meet at a right angle : the distal surface is 

 rounded off" transversely, and is rough, like the terminal phalanx of a multungu- 

 late quadruped : and there is not the slightest trace of any other joint than that 

 to which the first phalanx is adapted. Thus the fourth finger of the Mylodon 

 has but two phalanges, wanting the terminal one which supports the claw ; the 

 second phalanx being modified like the third phalanx of the same toe in the 

 Hippopotamus, which is enveloped in a callous hoof. The atrophy of the fifth 

 finger has proceeded to a greater extent : a small, simple oblong bone is 



* This character'has not been expressed sufficiently strongly in the plate. 



