INTRODUCTION 



the French observers have reached much the same conclusion regarding the 

 corresponding horizons in France, and are tending to enlarge the boundaries 

 of the Oligocene. The White River beds correspond very closely indeed to 

 the horizon of Ronzon, which is so generally taken as the type of the Middle 

 Oligocene, while the Uinta corresponds, somewhat less accurately, to the Paris 

 Gypsum (Lutetian) ; the latter is now referred by many to the Lower Oligocene, 

 and the Uinta should receive the same reference, for the two are so nearly 

 synchronous that they cannot well be separated into different periods. 



[Before taking up the principal subject of this paper, the Uinta seleno- 

 donts, it will be serviceable to give a revised account of certain White River 

 genera, especially Leptomeryx and Hypertragtilus, which will be found neces- 

 sary for the full comprehension of the Uinta forms. The new material, which 

 necessitates the revision and modification of several statements in my former 

 papers ('91, a, b, c ; '95, a, b), was obtained at various times by Messrs. Hatcher, 

 Gidley and Wells in the White River beds of Nebraska and South Dakota.] 



Geological Museum, Princeton, N. J., 

 December i, i8g8. 



