36 SCOTT. [Vol. V. 



times broader than that for the lunar, the proportion being 

 3 to i ; in Pcebrotherium it is as 9 to 5. Distally, the unciform 

 is taken up, for the most part, by the large surface for the 

 fourth metacarpal, but in addition there is a small but distinct 

 facet at the inner angle for a projection of mc. Ill, and on the 

 external side another and still smaller one for the rudimentary 

 fifth digit. Both of these facets are, however, lateral rather than 

 distal. The posterior process is well developed and forms a 

 stout decurved hook, which is relatively longer than in the 

 llama. 



The metacarpus (PI. II, Figs. 36, 37) consists of two func- 

 tional digits, the third and fourth, with small scale-like rudi- 

 ments of the second and fifth, all of them separate. Mc. II is 

 very short, with a somewhat enlarged and thickened head, but 

 tapering rapidly to a blunt point ; it lies in an excavation in 

 the radial side of mc. Ill, and is very closely applied to but not 

 co-ossified with it. The proximal end is convex, and shows two 

 very small facets, the larger of which is posterior and articulates 

 with the trapezium, while the anterior one articulates with 

 part of the trapezoid, but is altogether excluded from the 

 magnum. 



Mc. Ill is considerably elongated, but has not reached the 

 relative length which occurs in the modern genera; its length is 

 to that of the humerus as 85 to 100, while in the modern types 

 the length is about 95, and in some individuals even exceeds 

 the length of the humerus. The shaft is very slender through- 

 out, especially in the distal third. The proximal portion of the 

 shaft is of trihedral section, with flattened ulnar side which 

 is closely applied to mc. IV, while the anterior surface is 

 rounded. Near the distal end the flattening of the ulnar side 

 disappears and the shaft becomes nearly cylindrical. The distal 

 ends of the median metacarpals show a slight tendency to 

 diverge, obviously the beginning of the peculiar structure 

 which characterizes the cannon-bone of the Camelidcs. The 

 proximal articular surface presents a large, slightly concave 

 facet for the magnum, and a much smaller, though very distinct 

 one, for the trapezoid. The head rises above that of mc. IV 

 and sends out a process which abuts against the unciform, and 

 thus mc. Ill articulates with three of the carpal elements ; 

 beneath this process there is a fossa which receives the fourth 



