256 REPORT UNITED STATES GEOLOGICAL SURVEY. 



The double vertical line in the foregoing table may be taken to repre- 

 sent the Rocky Mountains, or the great range that extends northward 

 through Colorado, Wyoming and Montana; the localities or districts 

 represented on its left being east, and those on its right west, of those 

 mountains. An examination of the table will show that this mountain 

 range has no paleontological significance as a geographical boundary 

 between those eastern and western localities of Laramie strata, because 

 the species range across it almost as freely as they do across the space 

 which separates any two or more of the others. Indeed, the great con- 

 trast that is presented between the fauna of the brackish-water beds of 

 the Laramie Group in Bear Eiver Valley and that of the great body of 

 the group elsewhere, as now known, is not marked by any now existing 

 physical feature, and what the real cause of that contrast was, yet re- 

 mains to be discovered. It is evident that the present hypsometric con- 

 dition of the North American continent has no direct relation to the 

 distribution of species in the strata of the Laramie Group, or in any of 

 the Cretaceous groups. 



It is a fact worthy of especial notice that not a single species of all 

 those that have been found in the brackish-water beds of Bear Biver 

 Valley, with perhaps the exception of a Physa, is identical with any that 

 have yet been found in any other Laramie strata ; those indicated in the 

 table as thus identified having been obtained from the upper Laramie 

 beds at the Evanston coal-mines, which are of fresh-water origin. An- 

 other significant fact is that those species which are thus identified are 

 pulmonate mollusks ; the species which differ most widely from other 

 Laramie forms being branchiferous mollusks. The natural inference 

 from this fact is that the modifying conditions which then existed in this 

 part of the continent produced their effect upon that portion of the in- 

 vertebrate fauna which inhabited the principal waters, leaving the land 

 and palustral fauna comparatively unchanged. 



Taking a general view of the species as represented in the foregoing 

 table it will be seen that the palustral pulmonates occur in all the dis- 

 tricts indicated, and that land-shells also are not uncommon. These 

 facts, together with the identity of species and types of those mollusks 

 in the various districts, indicate great uniformity throughout the 

 whole Laramie period of such physical conditions as would affect 

 those mollusks. In considering the distribution of the other types 

 represented in the table, namely, those of the branchiferous mollusks, 

 for reasons already given, those of the brackish-water beds of Bear 

 Biver Valley must be, at least in part, excluded. We find, however, 

 that the Unionidce, CeriphasiidWj and Viviparities, among fresh-water 

 types, and the Ostreidw, Anomiidw, Cyrenidw, and Corbulidce, among 

 brackish- water types, are common to all the districts represented, the 

 Cyrenidw being especially numerous in species in Eastern Colorado. Be- 

 sides these, there are other types belonging to both categories which, so 

 far as is now known, are less widely distributed, but those families just 

 mentioned are sufficient to serve as a basis for some general remarks 

 which are to follow. So far as may be seen from the foregoing table, or 

 from any similar tabular exhibition of species, they may have occurred 

 promiscuously associated in the same layers at any and all of the locali- 

 ties indicated. On the contrary, certain of these types are, as a rule, 

 confined locally to certain layers, which respectively represent the ground 

 of their former habitats ; but there is not unfrequently found such an 

 admixture of types in one and the same layer as to show plainly that 

 some of them must have been drifted to the places of their present en- 

 tombment and association. 



