THE OSTEOLOGY OF ELOTHERIUM. 323 
blances in skull and dentition indicate any relationship between the two families can be 
determined only when their history has been worked out. In any event, it is not prob- 
able that the relationship can prove to be closer than that both lines were derived from a 
common stock which separated from the other Artiodactyla at a very early date. 
As has already been observed, no direct ancestors of Hlotheriwm haye yet been 
recovered, but there are certain Eocene forms which seem to be related to these unknown 
ancestors in such a way as to suggest the character of the latter. The Achawnodon 
(Elotherium) uintense of Osborn (95, p. 102) is such a form and differs from the 
A. robustum of the Bridger in the “ great elongation of the face and the shortening of the 
cranium, both of which characters relate it to Elotherium” (/. ¢., p. 103). This species 
is more specialized in several respects than the White River Elotheres, and like its fore- 
runners of the Bridger, A. robustum and A. insolens, it has but three premolars in each 
jaw, and hence is not at all likely to be ancestral to the later genus. In the Wasatch 
Achenodon is represented by A. (Parahyus) vagum Marsh, which likewise has but three 
premolars, and, so far as it is known, differs from the Bridger species only in its smaller 
size. There is some reason to think, as Osborn has pointed out, that even A. wintense had 
four functional digits. 
While it is very unlikely that Achwnodon can haye been the direct ancestor of Hlothe- 
rium, there are, nevertheless, so many suggestive resemblances between the two genera, and 
the types of their dentition are so nearly identical, that we can feel little doubt as to their 
real phylogenetic relationship. In this case, Achenodon will represent a somewhat modi- 
fied side-branch of the stem which culminated in Elotherium. <A species of Achenodon, 
or of some closely allied genus, with unreduced dentition and unshortened face, may well 
prove to be the desired ancestral form. If so, the line had already become distinct in the 
Wasatch and the group thus has no subsequent connection with any existing artiodactyl 
family, unless possibly with the Hippopotamide. Elotherium would then represent the 
termination of an ancient and very peculiar line, which attained a remarkable degree of 
specialization in many parts of its structure and which extended its range over the whole 
Northern Hemisphere. At the same time, the cerebral development of the genus was 
very backward and this was doubtless one, at least, of the factors which led to its extinc- 
tion. After the John Day, the line disappeared, leaving no successors. 
LITERATURE. 
48. Aymard, A. Coll. id. Mém. Soc. Agric. Sci. et Bell. Lett. du Puy., Vol. XII, 1848. 
79. Cope, EH. D. Observations on the Faun of the Miocene Tertiaries of Oregon. Bull. U.S. Geol. and Geogr. Survey 
of the Territories, Vol. V, No. 1. - 
74. Kowalevsky, W. Monographie der Gattung Anthracotherium. Paleontographica, Bd. XXII. 
