SCOTT: TOXODONTA OF THE SANTA CRUZ BEDS. 
159 
unciform and on the ulnar side is another facet, no doubt for the ves- 
tigial fifth metacarpal. 
In Toxodon the pyramidal is very similar to that of Nesodon, but is 
relatively broader and shorter, so that its proximal end is at a much 
lower level than that of the lunar and scaphoid, a difference which is 
much greater than in the Santa Cruz genus. On the ulnar side there is 
no facet for the rudiment of the fifth metacarpal. 
The pisiform is a short and heavy bone, not unlike a calcaneum in shape. 
The proximal end is transversely expanded and bears two irregularly tri- 
angular facets, for the pyramidal and ulna respectively; the latter is con- 
siderably the larger of the two and more concave. The body of the bone 
is short, heavy, laterally compressed, and has a strongly convex ventral 
border, while the distal or free end is much thickened and rugose. 
In Toxodon the pisiform is of quite a different shape, being much 
deeper dorso-ventrally, thinner and more compressed laterally, and is not 
nearly so much expanded at the proximal end. 
The trapezium is very small and has no direct articulation with the sca- 
phoid or even with the trapezoid. It is an irregular, nodular bone, with 
the principal diameter in the proximo-distal direction, and having rough- 
ened ends. On the ulnar side is a relatively large, subcircular and con- 
cave facet for me. II and on the palmar side is a very small facet, the pur- 
pose of which is doubtful, but may have been for the attachment of a 
sesamoid. Ameghino (’94*, 246, fig. 1) regards this bone as the vestigial 
me. I and it is quite possible that this is the correct interpretation, as is 
suggested by the absence of any articulation with the scaphoid. In that 
case, the minute facet on the palmar side might be for an even more reduced 
trapezium. From the analogy of other ungulates, however, it seems much 
more probable that the bone in question is the trapezium, which almost 
invariably persists as a relatively large element long after all trace of the 
pollex has disappeared and is itself suppressed only in a few highly spe- 
cialized types with monodactyl or didactyl feet. So far as can be judged 
from the cast of the La Plata skeleton, this problematical element, 
whether trapezium or first metacarpal, would seem to be lacking in 
Toxodon. 
The trapezoid is a relatively large and important bone, with subquadrate 
dorsal face. Toward the palmar side the bone narrows much and is thus 
wedge-shaped. Its position in the carpus is somewhat oblique, as it faces 
