170 THE AGE OF MAMMALS 



western America. The Equidse are represented by Epihippus, the very 

 diminutive horse in a stage of evolution which is not quite so advanced 

 as that of Lophiotherium cervulum of the gypse, or Ludian, stage of France. 

 The tapirs (Is'edolophus) and rhinoceroses (Amynodon) occur, but in general 

 we observe that forest and fluviatile, or river-frequenting, forms are rare 

 in this formation. This explains perhaps why the bunodont or omnivorous 

 entelodonts, or elotheres, have not been found. The conclusion is that 

 the conditions of deposition and fossilization at this time were less favor- 

 able to the collection of river and swamp dwellers, and more favorable 

 to the preservation of the upland and meadow or field Herbivora. 



For the first time in North American history the Artiodactyla of the 

 ruminant division, or Pecora, begin to abound, herbivores of diminutive and 

 intermediate size, with short-crowned molar teeth of the crescentic or 

 selenodont pattern. The Diohobunidse, or Homacodontidse, are represented 

 by Bunomeryx. Most interesting is the rise of the camel family. Thus 

 the Camelidae, or Hypertragulidae, are represented by four genera, Lepto- 

 tragulus, Protylopus, Camelomeryx, Oromeryx. Of these, the diminutive 

 Protylopus has been selected as the possible ancestor of the grand phylum 

 of American camels. The polyphyletic tendency in this family is already 

 displayed, and undoubtedly more than one line is represented here. The 

 Oreodontidse are similarly abundant, embracing ancestors both of the 

 typical oreodonts {Protoreodon) and of the aberrant agriochoerids {Pro- 

 tagriochoerus) . 



Among the carnivorous enemies of these Herbivora were the smaller 

 creodont oxyaenids and the giant Mesonyx, as well as the smaller fox-like 

 canids or pro-Carnivora, Miacis and Procynodictis. This is the last ap- 

 pearance in North America of these two creodont families (Oxyaenidse and 

 Mesonychidse), and there is some reason for thinking that the American 

 Hyaenodontidae (Sinopa) had already become extinct and that the hyseno- 

 donts which we shall find appearing in the Lower Oligocene were invading 

 forms from the Old World or from the north. If this proves to be the 

 case, it may be said that the Upper Eocene of America is marked by the 

 final disappearance of all the archaic herbivorous and 

 carnivorous mammals of American residence as well as by 

 the incipient extension of the great order Artiodactyla. 



The Atlantic Coast Region 



The Zeuglodon Zone. — The Zeuglodon Beds of the southeastern states 

 are referred to the Jackson ^ Formation, which is regarded as the middle 



* Lyell, 1847, was the first to assign the Zeuglodon beds to the Jackson Formation below the 

 Vicksburg; this was recognized by Hale, and strongly insisted on by Hilgard (1867). The Jack- 

 son was regarded by Dana (1895) as Middle Eocene, approximately equivalent to the Bridger 

 Formation in the Rocky Mountain region. 



