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 Elotlierium have 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 Achcenodon 

 {Elotlierium) uinteme of Osborn ('95, p. 102) is such a form and differs from the 

 A. rohustum of the Bridger in the " great elongation of the face and the shortening of the 

 cranium, both of which characters relate it to Elotlierium" (I. c, p. 103). This sjjecies 

 is more specialized in several respects than the White River Elotheres, and like its fore- 

 runners of the Bridger, A. rohustum 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 

 Achcenodon 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. uintense had 

 four functional digits. 



While it is very unlikely that Achcenodon can have been the direct ancestor of Elotlie- 

 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, Achcenodon will represent a somewhat modi- 

 fied side-branch of the stem which culminated in Elotlierium. A species of Achcenodon, 

 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 Hippopotamidce. Elotlierium 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. Mem. Soo. Agrie. Sci. et Bell. Lett, du Puy., Vol. XII, 1S48. 



'79. Cope, E. D. Observations on the Fauna; of tbe Miocene Tertiaries of Oregon. Bull. IT. S. Geol. and Geogr. Survey 



of the Territories, Vol. V, No. 1. 

 '74. Kowalevsky, W. Monographie der Gattuug Anthracotherium. Palaeontographica, Bd. XXII. 



