6 DINOCERATA. 



largest and most characteristic genus of wliich is the ungulate Coryphodon, 

 and hence the author has called these deposits the Coryphodon beds. The 

 middle Eocene strata, which have been termed the Green River and 

 Bridger series, has been designated by the author the Dinoceras beds, as 

 the gigantic animals of this order are only found here. It is, however, 

 better to separate the Green River series, under the term Heliobatis beds, 

 and this is done in the present volume. The name Dinoceras beds will then 

 apply to the Bridger series alone. The uppermost Eocene, or the Uinta 

 group, is especially well characterized by large mammals of the genus 

 Diplacodon, and hence termed by the author Diplacodon beds. The fauna 

 of each of these three subdivisions was essentially distinct, and the fossil 

 remains of each were entombed in diflferent and successive ancient lakes. 



It is important to remember that these Eocene lake-basins all lie 

 between the Rocky Mountains on the east and the Wasatch Range on 

 the west, or along the high central plateau of the continent. As these 

 mountain chains were elevated, the inclosed Cretaceous sea, cut off from 

 the ocean, gradually freshened, and formed these extensive lakes, while 

 the surrounding land was covered with a luxuriant tropical vegetation, 

 and with many strange forms of animal life. As the upward movement 

 of this region continued, these lake-basins, which for ages had been filling 

 up, preserving in their sediments a faithful record of Eocene life-history, 

 were slowly drained by the constant deepening of the outflowing rivers, 

 and they have since remained essentially dry land. 



The Miocene lake-basins are on the flanks of this region, where only 

 land had been since the close of the Cretaceous. These basins contain 

 three faunas, nearly or quite distinct. The lowest Miocene, which is 

 found east of the Rocky Mountains, alone contains the peculiar mammals 

 known as the Brontotheridce, and these deposits have been called by the 

 author the Brontotherium beds. The strata next above, which represent 

 the middle Miocene, have as their most characteristic fossil the genus 

 Oreodon, and are known as the Oreodon beds. The upper Miocene, which 

 occurs in Oregon, is of great thickness, and from one of its most important 

 fossils, MioMppus, has been designated as the Miohippus series. The 

 climate here during this period was warm temperate. 



