58 



CEXOZOIC MAMM-AX HORIZONS 



in the Great Plains. How long previously these conditions had set 

 in it is impossible to say. In the entire Oligocene and Pliocene beds 

 thus far only 6 species of water-Hving turtles have been described or 

 recorded, and these are probably from river-channel sandstones, as 

 compared with a very much larger number of land-hving tortoises. 



The details are as follows: (1) In the White River group Gower 

 Oligocene) there occur 8 species of the Testudinidae. including one of 

 the land tortoise St]/lemi/s: one species of Testudo, T. hronfops Marsh, 

 belongs to the Chadron formation, or Tifanotherium zone: all of these 

 are land tortoises, mostly found in Colorado. Of water-hving forms 

 the White River group of South Dakota has furnished one species of 

 the Emydidie. river turtles, and one of the Derniatemydidse. a small 

 family related to the Chelydridae and now confined to Central America. 

 (2) In the raiddle part of the John Day formation (upper Oligocene, 

 Moimtain Region) there are 3 species of Sfi/lemys. land tonoises. (3) 

 In the Deep River sequence (middle Pliocene, Mountain Region) 

 occiu^ a single species of Testudo: from the Mascall formation of 

 Oregon there is known a species of Clemtnys, a genus now hving in 

 America and Asia. From the deposits on Pawnee Creek. Colorado, 

 come 2 large species of Testudo. (I) The 'Loup Fork beds" (upper 

 Pliocene. Plains Region^ furnish 7 species of Testudo. approaching in 

 size the great tortoises of the Galapagos Islands, (oj From the Rat- 

 tlesnake formation of Oregon (PHocene) occurs a species of Clemmys, 

 a land tortoise. 



Modenuzation in Xorth America. — A second American moderniza- 

 tion, as remarkable as the first or Wasatch modernization, is shown 

 by the first appearance of 16 famihes of mammals which have not as 

 yet been certainly recognized in the Moimtain Eocene basins, namely, 

 6 existing families of Rodentia, 4 existing families of Camivora, 4 

 existing famihes of Insectivora. 1 existing family of Perissodactyla, 

 1 now extinct European family of Artiodactyla. 



Mrxlernization in Europe. — A very similar modernization occurred 

 in western Europe.*^ In the Ludien ( = lower Oligocene, Lapparent, 

 uppermost Eocene, Deperet). Sannoisien, and Stampien ( = lower Oh- 

 gocene) 17 modem or still existing families which have not been 

 foimd in earher geologic stages appear for the first time. Of these 

 new famihes, 6 appeared simidtaneously in Xorth America. 



a This generalization is chieflr based upoa the faimal lists of Deperet (see footnote, p. 9). 



