24 Bulletin American Museum of Natural History. [Vol. XII, 



the species to each other are not yet known. Leptauchenia is un- 

 known in the John Day basin, and is very close to the Upper 

 Miocene Cyclopidius, but if these genera were aquatic they may 

 have had a peculiar and limited habitat, and their absence would 

 not have much weight. 



The so-called Loup Fork Beds are not all strictly of the same 

 age. The more easterly and southerly deposits in Nebraska, 

 Kansas, and Texas are uppermost Miocene or Pliocene. The 

 species described by Leidy and Marsh came chiefly from these 

 beds. The Colorado Loup Fork is a distinctly older horizon, 

 Upper or perhaps in part Middle Miocene. Prof. Cope explored 

 this region in 1873 and 1879 and described its fauna. He seems 

 to have been less familiar with the eastern Loup Fork ; and this 

 may partly explain the discrepant views held as to the age of the 

 Loup Fork Tertiary. Williston and some other recent writers 

 entirely refuse to assign a definite age to the Kansas Loup Fork ; 

 and the present writer desires to avoid including with the beds to 

 which a definite age is assigned, any but those in which a suf- 

 ficient" fauna has been found. The Colorado Loup Fork is here 

 placed as equivalent to the Deep River of Montana, the Ticho- 

 leptus Beds of Cope — an unfortunate name, as Ticholeptus is a 

 synonym of Merychyus, and Merychyus is equally characteristic of 

 the typical Loup Fork or Procamelus Beds. Cope placed with 

 the Deep River the Upper Miocene Beds of the John Day valley, 

 and of Laramie Peak, Wyoming. Other beds on the western and 

 northern margin of the Great Plains basin may be equivalent. 

 Scott considers the Deep River as older than any of the beds 

 above mentioned, laying especial stress on the occurrence of 

 Cyclopidius in that basin only. Cyclopidius^ however, occurs in the 

 extreme west of Nebraska (Cheyenne Co.), and its absence else- 

 where may be explained, as above noted, by its aquatic habitat. 

 Miolabis ' characterizes the Colorado Loup Fork and the Oregon 

 beds, and stands well in opposition to Procamelus of the upper 

 horizon. The lower beds have been called Cyclopidius Beds.° 

 They show the following characteristic faunal differences : 



^ ( = Protolabis Wortman, 1898, not Cope.) 



* On account of its evident inconstancy a name derived from the fauna to designate a par- 

 ticular horizon is less suitable than a geographic term. Pawnee Creek Beds would be a better 

 name for the lower horizon in Colorado, Deep River being used correlatively with Loup Fork 

 for the general designation of the lower beds. 



