NO. 8 UPPER EOCENE ARTIODACTYLA — GAZIN 1 5 



of Leptotragulns. Scott (1899) regarded Leptoreodon in this rela- 

 tionship, but the premolars in this form are rather distinctive and not 

 like those in Protoceras, Instead, Scott indicated with a query that 

 Hypertragidus may have come from Leptotragnlus, but this suggestion 

 is likewise unacceptable. It may be further noted in the chart presented 

 here that Peterson's Poabromylus is removed from the Camelidae 

 and tentatively aligned with Leptoreodon. This, of course, is not 

 certain, but I am convinced from the molar structure that it is 

 leptotraguline rather than camelid. 



Reviewing the various Eocene artiodactyls of North America, with 

 the amazing array of Old World forms in mind, one is impressed by 

 the resemblance of certain North American groups to others in the 

 European assemblage. The division of the dichobunids has been 

 noted, and the similarity between early Eocene Hexacodus and Proto- 

 dichohiine is possibly the closest actual approach between the two 

 major portions of the family, although these genera appear to have 

 distinctive premolars. In the middle Eocene, Homacodon is rather 

 like Dichohune, though with simpler premolars, and perhaps like 

 Mouillacitherhim, but again with simpler premolars and in addition 

 a better-developed protoconule on the upper molars, to judge by 

 Stehlin's ( 1906) excellent illustrations. Some of the earlier Dichobune 

 species, however, with a weaker hypocone are more suggestive of 

 Helohyus. It should be noted, moreover, that much of the European 

 middle Eocene dichobunid material shows a precocious tendency to- 

 ward selenodonty not seen in any of our Bridger forms. Furthermore, 

 the more-distinctive selenodont groups such as the anthracotheres and 

 anoplotheres also appear in the middle Eocene assemblages of 

 Europe, and a comparable though independent development in North 

 America is not seen until upper Eocene. 



Although there is a rough similarity between our upper Eocene 

 selenodonts and those of Europe, as for example between the agrio- 

 choerids and anoplotheres, or between our camelids and the xipho- 

 donts, there are fundamental differences between these groups, and 

 I fail to find any direct line between them or any justification for 

 considering our selenodont stocks as derived from those of Europe. 

 The trend toward selenodonty has surely progressed independently, 

 in parallel though not strictly identical ways, in the two areas. More 

 adequate information on the origin of the North American selenodont 

 groups will possibly be revealed when faunas of Uinta A time or 

 intermediate horizons in the Washakie are discovered. It is highly 

 probable that no interchange of faunas took place between the two 

 continents from about the beginning of the Eocene to its end. There 

 is, however, as at the beginning of the Eocene, ample evidence for 



