616 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



representatives of all of them may yet be discovered in the great 

 lacustrine deposits of the West. It is possible also that some of the very 

 few groups that are now known to be represented only by fossil forms 

 may yet be found to be represented by living species in some part of the 

 continent. But this is less probable, and we may doubtless safely assume 

 that most, if not all of them, have become extinct with the extinction of 

 the few species that composed them. 



The catalogue comprised in Paleontological Papers Ko. 3 shows how 

 numerous are the species already discovered in those western North 

 American strata, none of which, not even the most recent, have yet been 

 referred to living species. But when this whole subject shall be taken 

 up for judicious synthetical study, it will probably appear that sufficient 

 cause for separating many of them from existing species, as well as from 

 each other, will be difficult to find. 



Although, as before stated, the present discussion is intentionally limited 

 to the true fresh-water and land Mollusks, it is proper to state that some 

 of these fossils have been found in such association with others that are 

 regarded as brackish-water species as to indicate that they were prob- 

 ably capable of living in water that was impregnated with at least a 

 small proportion of salt, which their living representatives seem incap- 

 able of enduring. For example, at a locality in Southwestern Wyoming, 

 in strata that are designated in the catalogue as Post-Cretaceous, two 

 species of Unio are abundantly represented in association with occasional 

 specimens of an Oyster and other Mollusks that also indicate a brackish- 

 water habitat. !N"one of these specimens present the appearance of hav- 

 ing been drifted or transported to their present position and association, 

 but all seem to have lived and flourished together. Also, in strata at 

 Black Buttes Station, Wyoming, that are a little more recent than those 

 just referred to, several species of the genera Unio and Viviparus are 

 found associated in the same layers with Corbula, Corbicula, Neritina, 

 etc., which layers alternate with others that coatain Ostrea and Anomia. 

 These facts, while they do not affect the validity of those which I pro- 

 jiose to discuss, suggest an important bearing upon the subject of the 

 early differentiation of fresh- and brackish-water Molluscan types, which 

 will be further considered on a subsequent page. 



The species embraced in the catalogue which present the greatest 

 differences from their related existing forms are regarded as true brack- 

 ish-water species, and therefore do not fall within the especial scope of 

 this paper. The few fresh- water species that are likewise not congeneric 

 with living forms are confined to strata that have hitherto been but 

 little studied, and are not yet correlated with those of any of the groups 

 of strata named in the tables preceding the catalogue. They are those 

 which in the catalogue are referred to the following localities : — Valley 

 of Snake Eiver, Southwestern Idaho ; Kahsow Mountains, Nevada; and 

 Crow Creek, Northern Colorado. Those among these few species that 

 are referred to the genus Melania differ considerably from all other fossil 



