724 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



mouy with the close faunal lelationship, which is shown to exist, by 

 the few species that are Darned in the table. 



The brackish- water branch iferous species, however (as well as the 

 pulmonote BhytopJiorus priscus Meek), of the Bear Eiver Valley series,^ 

 are not only of different species from any that occur in any other strata 

 of the Laramie Group, but a part of them are of diiferent types also. 

 It is also true that these brackish- water species depart more widely from 

 living types than do any of the species of other portions of the Laramie 

 Group. In fact, not one of the species yet found in the true brackish- 

 water strata of the Bear River series has been identified in those of any 

 of the other regions discussed in this pai)er ; and the evidence of the 

 faunal relationship of this portion of the Laramie Group with the others, 

 which is shown in the table, is confined to pulmonate mollusks alone. 

 It is true also that the pulmonate mollusks of the Bear River Valley series 

 that have been identified with species found in Laramie strata in other 

 districts are apparently confined to the Evanston coal-bearing beds 

 that overlie the portion of the series in the Bear River Valley which 

 contains the brackish-water types. The fact that these pulmonate 

 species of the Evanston coal-bearing beds have also been found only in 

 the Judith River series, which probably represents the lower or earlier 

 portion of the Laramie Group, seems to indicate that the Bear River 

 series of brackish-water strata is still older. But this is not necessarily 

 the case, for there is apparently no reason why we might not expect to 

 find those species to range through the whole Laramie series, as other 

 species have been shown to do. In other words, from our present 

 knowledge of the facts, it appears justifiable to regard the Judith River 

 beds as representing the earlier and the coal-bearing beds near Evans- 

 ton as the later portion of the Laramie period. 



It now seems probable that we must look for the cause of the differ- 

 ences which the branchiferous mollusks of the strata of the Bear River 

 Valley present, from all other portions of the Laramie Group, in a differ- 

 ence of physical conditions probably induced by the proximity of the 

 western shore-line of the great Laramie inland sea ,• conditions that 

 induced differential changes in the aqueous mollusks, but not thus 

 affecting the land and palustral pulmonates. 



In subsequent papers, it is proposed to discuss the relations of the 

 Laramie Group with those above and beneath it; and also the relations 

 of its molluscan types with those of other fossil, and also with those of 

 existing forms. 



