OEETACEOUS FOSSILS. 163 



FOSSILS OF THE BEAR RIVER FRESH- OR BRACKISH- 

 WATER BEDS. 



As it is still a matter of some doubt whether the above-mentioned 

 highly-inclined strata seen on Sulphur Creek near Bear River, Wyoming, 

 very nearly conformable to well-marked marine Cretaceous beds at the 

 same locality, belong to the latest member of the Cretaceous or to the earliest 

 Eocene Tertiary, I prefer to describe the fossils from them here separately 

 under a distinct heading. I have from the first inclined to the opinion that 

 these brackish-water beds belong to the horizon of the oldest Eocene, though 

 I have several times mentioned certain reasons for suspecting that they may . 

 prove rather to belong to the closing period of the Cretaceous. One diffi- 

 culty in deciding in regard to their precise horizon is, that all of the fossils 

 yet known from them are new and entirely distinct from those found else- 

 where in very well- determined horizons. None of them belong to any of 

 the characteristic Cretaceous genera, while several of the species are nearly 

 allied to Lower Eocene forms of the Old World, though clearly distinct 

 specifically. 



The fact, however, that at least one species occurring here seems to be 

 identical with a form {Vivipara Conradi) found in a similar brackish-water 

 group of strata, associated with Cretaceous types of Vertebrate remains at 

 the mouth of Judith River on the Upper Missouri, when taken in connec- 

 tion with the recent discovery of a Cretaceous type of Saurian remains in 

 Wyoming, apparently at a higher horizon, and of a decidedly brackish- 

 water group of MoUuscan remains beneath a considerable thickness of Cre- 

 taceous strata at Coahdlle, Utah, certainly seems to indicate that these beds 

 belong rather below than above the line of division between the latest Cre- 

 taceous and the oldest Eocene. Unfortunately, however, as I have often 

 remarked, fresh-water and estuary shells do not generally present the same 

 well-defined distinguishing features, if I may so speak, in each of the vari- 

 ous geological horizons, usually observable among marine types, being 

 often very similar from rocks of quite different ages, and again quite distinct 

 in equivalent beds at different localities. 



