LIV INTRODUCTORY REMARKS. 



» 



the fossils of this series, and the evidences of the greater saltness of the 

 waters in which many of these beds were deposited, we have much stronger 

 reasons for believing that they, at least in part, represent the Judith River 

 group. 



After visiting these Wyoming localities in 1873, and seeing how the 

 same general types of fossils range through the whole, from the heavy beds 

 of lead-gray and yellowish sandstones along Bitter Creek near Rock Spring 

 and Point of Rocks, far up through the strata including the Hallville coal- 

 beds, to those at Black Butte station, in which the Dinosaurian and other 

 fossils were found, the author was led to believe that they belong to one 

 great series, nowhere divisible on any well-defined paheontological evidence. 



This fact, and the presence of the Dinosaurian far up near the top of 

 the series, rather inclined the author to the opinion that the whole belongs 

 to the Cretaceous ; but he did not regard the evidence as altogether con- 

 clusive, and mentioned the entire absence, so far as then known, of any of 

 the peculiarly characteristic Cretaceous genera of Mollusca in these strata.* 



Since that time, .Dr. White and Professor Powell have found in one of 

 the lower beds near Point of Rocks — the same as that from which the 

 Anovtia was obtained that was regarded by the author as scarcely distin- 

 guishable from a Texas Cretaceous species described by Dr. Roemer — a fine 

 species of a new genus of Buccinoid shells described in this work under the 

 name Odontobasis.\ The only other two species of this peculiar genus 

 known (including the type) were found in the Fort Pierre group of the 

 Upper Missouri Cretaceous, where they occur associated with strictly marine 

 forms. The presence here of a species of this peculiar genus— elsewhere 

 only known in the Cretaceous — furnishes another strong argument in favor 

 of the conclusion that these rocks belong to that epoch. 



Some four hundred feet or more above the horizon at which this fossil 

 and the Anomia and an Ostrea that the author had originally arranged in the 

 Cretaceous list occur, Professor Powell and Dr. White draw the line between 

 the Cretaceous and Tertiary of this region ; leaving the Hallville beds con- 

 taining the two fossils that had originally been referred by the author to the 

 Lower Eocene, and all the beds above, up to and including the Dinosaurian 

 bed at Black Butte, in the Lower Tertiary. They draw this line, however, 

 entirely on stratigraphieal evidence, there being something of a physical break 



* Auu. Eoi>_. U. S. Geol. Survey for 1873, 459. I See i':iye :551. 



