WHITE ON PALEONTOLOGY. 623 



to be without anj' aiore evidence of immediate relationship to marine 

 species than that which the fauna of the present North American rivers 

 possesses, although it may be assumed that primarily all Molluscan life 

 must have originated in mariue waters. 



Every person familiar with the numerous species of Unio now living 

 in the streams of the great Mississippi Eiver system knows that differ- 

 ent portions of the same stream are found to afford habitats for certain 

 species that are more congenial to them than they are to other species. 

 Thus the experienced collector expects to find certain species, among 

 which are those of the more differentiated forms and thicker test, in the 

 swifter waters and upon gravelly or rocky bottom, while other species 

 ■will be found on sandy or muddy bottom and in stiller waters ; and 

 these latter species have usually the smooth or plain surface, thinner 

 t3St, and oval or approximately oval form. As a rule, also, the Unione, 

 as well as the other Molluscan, fauna of the lakes and ponds of North 

 America is remarkably meager compared with that of the running 

 streams. This may be in part due to the fact that the great lakes, and 

 many of the smaller ones also, belong to the great Saint Lawrence River 

 system, which, as before stated, possesses a comparatively meager Uni- 

 oue fauna. 



But that this result is not wholly due to that cause is shown by the 

 fact that the lakes and ponds which empty into streams that are rich 

 in Vnionidcc themselves contain comparatively few species, and these 

 species almost always possess the same characteristics that those do 

 which are found in the stiller portions of the streams into which the 

 lakes overflow. 



With these facts in mind, we should naturally expect to fin'd the re 

 mains of a meager Unione, as well as other Molluscan, fauna in those 

 great lacustrine Tertiary deposits of Western North America ; and to 

 find also the Unionidce represented there by species having a smooth sur- 

 face and oval or approximately oval form. This is exactly what is found 

 to be the case in all those strata which bear evidence of having been de- 

 posited in purely fresh waters ; but in those strata that bear evidence 

 of having been deposited in waters that were a little salt, the Unionidce 

 are much more differentiated. Indeed, it is in the last-mentioned strata 

 alone that those species of Unio have been found that possess the pecu- 

 liar North American characteristics. 



It is well known that the maximum of differentiation, of Molluscan 

 types takes place in marine waters ; that it is much less in brackish 

 waters ; and that the minimum in this respect is reached in purely fresh 

 waters. Judging from facts before stated, it would seem that those 

 ancient Unionidce were not only capable of living in waters that were a 

 little salt, but that the influence of the salt upon them was such as is 

 in a general way exerted by it upon all Molluscan life, producing a 

 greater differentiation than would have been produced in fresh lacus- 

 trine waters, and such as has generally supposed to have been exerted 

 upon the family in existing fluviatile waters. While it is not unreason- 



