228 



THE AGE OF MAMMALS 



fication. In the latter part of the period the topography appears to have 

 been more diversified and the action of streams to have become more 

 pronounced (p. 3). The Mollusca are terrestrial, or air breathing types, 

 with the exception of those of one locahty, which are fluviatile. The 

 Testudinata (genus Stylemys) are of the Testudo or terrestrial type, no 

 river-living turtles having been recorded ; neither is there any evidence 

 of crocodiles. Even the so-called beavers (Castoridse) are of the genus 

 Steneofiber, not a true river-living form. This time-keeping rodent (see 





AUur, 



Fig. Ho. \ ifw ut Srott's Hlulf, a fainoiis laiicliiiaik iu western Nebraska. L iiin-r * )li!io- 

 cene of the Oreodon and supposed Leptauchenia Zones, overlaid by Miocene. Photograph by 

 American Museum of Natural History, 1908. 



p. 197), it may be observed parenthetically, is in the same stage of evo- 

 lution as its relatives in St. Gerand-le-Puy of France. The remains of 

 forests are found in the middle of the tuff deposits, and the great forest 

 at the summit was overtaken and submerged by the lava flow, the trunks 

 of the trees still standing. 



The known mammalian fauna of the John Day Formation is chiefly 

 of the open-forest, river-border, and savannah-living type. Brachyodont 

 or browsing types of molar teeth still prevail. The beginning of this great 

 deposition in the John Day valley of Oregon appears to correspond very 

 nearly with that of the closing deposition of the White River group, Brule 

 Clays, or Leptauchenia Zone of the Dakota region just described; but 

 the mammals of these beds are so sparse and little known that few deduc- 



