420 THE SUCCESSION OF THE VERTEBRATE FAUNAS. 



Eocene. 



The succession of three gradually differentiating mammal- 

 faunas in strata of the Eocene series, is determined by strati- 

 graphical geology both in Europe and North America. In 

 north-western Europe many of the strata containing them are 

 intercalated in a marine series which can be traced over a 

 considerable area. In North America, they are contained in 

 three successive lake-deposits, which happen to overlap in parts 

 of Utah and Colorado, where their order of formation is clear. 



The Lower Eocene fauna of Europe is represented in the 

 Thanet Sands, London Clay, and Woolwich and Reading Beds 

 in the south of England, also in corresponding formations in 

 the Paris Basin. In North America it occurs in the lacustrine 

 Wasatch Formation of New Mexico, Utah, Colorado, and 

 Wyoming. The marsupials are still represented by opossums 

 (Didelphys) on both continents. Compared with those of the 

 basal Eocene, the Creodonta and Condylarthra are now more 

 specialized and include many larger animals. Of the second 

 sub-order just mentioned, Phenacodus is the most important 

 and best-known genus. The true Perissodactyla are now 

 represented among the ungulates, by ancestral types of horses 

 (Hyracotherium}, rhinoceroses (Heptodon), and tapirs (Syste- 

 modon). A few Artiodactyla also begin to appear; but the 

 most characteristic ungulates of the period are the Amblypoda, 

 represented especially by Coryphodon. Undoubted lemuroids 

 and rodents likewise appear for the first time. 



The Middle Eocene fauna is very imperfectly known in 

 Europe, being found only in the marine Bracklesham Beds of 

 Sussex and in various freshwater deposits in France and Alsace. 

 In North America it comprises a large series of mammalian 

 genera discovered in the lacustrine Bridger Formation of 

 Wyoming and Colorado. All the groups of mammals are 

 more specialized than previously, and among the ungulates 

 the sub-orders of Perissodactyla and Amblypoda attain a 

 remarkable development in North America. On that conti- 

 nent the large Titanotheriida? begin to appear (Palceosyops), 

 while Coryphodon has become extinct and been replaced by 



