THE STRUCTURE AND RELATIONSHIPS OF ANCODUS. 
479 
the trapezium, which is lower and slightly concave in front, higher and somewhat 
convex behind. The shaft is compressed in its proximal part, but of considerable 
dorso-palmar depth ; it tapers inferiorly, expanding again to the distal end. The 
distal trochlea is well-developed and of almost hemispherical form, with a carina 
which is confined to the palmar side. It is obvious that this metacarpal must have 
been provided with both phalanges. The pollex of Oreodon, and even of the earlier 
and more primitive Protoreodon, is considerably more reduced and slender than that 
of Ancodus, and it is surprising to find a White River genus, so advanced in many 
respects, retaining such an ancient character in so perfect a way. 
Kowalevsky did not suspect the possibility of the existence of a pollex in 
Ancodus, or, indeed, in any artiodactyl, and hence his figures of me. II do not show 
whether it had the facets for me. I, which occur in the American species. 
The second metacarpal is much larger in every dimension than the first, and 
has about the same relative development as in Oreodon. The head is narrow, and 
is excavated on the radial side to receive the head of me. I, for which it has distinct 
facets. It is also in contact, to a slight degree, with the trapezium. The trapezoid 
surface is narrow and oblique, rising toward the ulnar side, concave transversely 
and slightly convex in the antero-posterior direction. The ulnar border of the 
proximal end overlaps the head of me. Ill, and abuts against the magnum by a facet, 
which is proportionately better developed than in Oreodon, and extends along the 
entire dorso-palmar depth of the head. The shaft is fiattened and compressed, but 
stout and considerably curved. The distal trochlea is rounded and prominent, and 
demarcated from the shaft on the dorsal face by a narrow depression. ^ Except for the 
larger size and greater prominence of the magnum facet, this bone is almost a copy 
of the corresponding one in Oreodon. The Ronzon specimens of me. II, figured and 
described by Kowalevsky, would appear to have a less prominent projecting process 
for the magnum and a straighter and more slender shaft. 
The third metacarpal is the longest of the series, rising above me. IV proximally, 
and descending below it distalij, but, on the other hand, it is a little narrower tans- 
versely than that bone. The head is broad and heavy; on the radial side, below 
the head, the shaft is excavated to receive an expansion of mo. II. There is a 
relatively large surface for articulation with the distal side of the projection which 
me. II sends to meet the magnunii this surface is divided into two parts by a nar- 
row sulcus. The posterior projection from the head is longer wider, and more 
massive in proportion than that of Oreodon, and the portion of the magnum face 
which extends upon it is broader and inclined mo« ‘“"<1 wA J 
main portion of the same facet is rather less strongly convex from 
low, confined to the palmar side, and hardly visible from the anterior side. 
