i6o 
PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
nearly as much toward the radial as toward the dorsal side of the manus. 
The proximal end forms a convex head, which fits into the concavity on the 
distal end of the scaphoid. The only other carpal with which the trape- 
zoid articulates is the magnum, for which there are two incompletely sepa- 
rated facets on the ulnar side, one proximal and the other distal. Distally, 
the large surface for me. II is saddle-shaped, concave palmo-dorsally and 
convex transversely. The trapezoid of Toxodon is very similar to that 
of the Santa Cruz genus, but proportionately larger and heavier. 
The magnum is quite large, especially in the proximo-distal dimension, 
exceeding the trapezoid in size, but is much smaller than the unciform. 
It has a very oblique position in the carpus, being inclined distally and 
toward the ulnar side. The ulnar face, which articulates with the lunar 
and unciform, is nearly plane and from the scaphoid to the head of me. 
IV there is a long, straight, oblique joint, between the magnum and me. 
Ill on the radial, and the lunar and unciform on the ulnar side. A some- 
what similar arrangement may be observed in the carpus of Ccenopus , a 
tridactyl rhinoceros from the Oligocene of North America. The magnum 
has a very complex arrangement of facets, articulating with all the other 
carpal elements except the trapezium, pyramidal and pisiform. The dorsal 
and palmar faces are strongly rugose and on the latter is a rough protuber- 
ance, but not the long hook so frequently seen in ungulates. 
As is usual, the magnum is much lower proximo-distally on the dorsal 
than on the palmar side, but the rise of the proximal surface is gradual and 
there is no well-defined head. On the radial side is the very large and 
complex surface which articulates with the process from the head of me. 
II ; this facet is quite strongly convex palmo-dorsally, except the much 
smaller palmar extension, which is concave. Above this facet is the small 
oblique, L-shaped surface for the trapezoid ; that for the scaphoid is large, 
transversely concave and dorsally occupies the whole width of the mag- 
num, but toward the palmar side this facet becomes narrower and more 
oblique. The lunar surface is quite distinctly divided into two parts ; the 
dorsal portion, which is slightly concave, is obliquely lateral in position 
and is continuous with the facet for the unciform, the two surfaces lying in 
nearly the same plane. The palmar portion of the lunar facet is convex 
and much more nearly proximal in position than the dorsal portion. The 
unciform surface is quite small dorsally and is continued as a very narrow 
band almost to the palmar border. Distally, the facet for me. Ill is large 
