WHITE.] GENERAL DISCUSSION. 485 



luscan fauna, which now so strongly characterizes them, until after the 

 confluence with them of the western portions of the present great river 

 system which brought that fauna from its ancient home in the western 

 part of the continent.* 



Eivers having been thus persistent, and the manner in which conflu- 

 ence of the waters of many of them has been effected being understood, 

 it is no more remarkable that the types of fresh-water gill-bearing mol- 

 lusca have come down to us from former geological periods practically 

 unchanged, than it is that marine and land mollusca have reached us 

 bearing the imprint of their really ancient, but what we have been ac- 

 customed to call, modern types. 



. The manner in which the various types of molluscan life have proba- 

 bly come down to the present time from former geological periods hav- 

 ing been pointed out, we come next to inquire to what extent the views 

 thus expressed are confirmed by a* comparison of the living with the 

 fossil non-marine mollusca. As regards the fresh-water and land mol- 

 lusca, it may be stated without hesitation that those views are fully 

 confirmed by such a comparison. That is, we find between the fossil 

 and living faunae such an extensive agreement of types as to compel 

 the conclusion that the former represent the latter ancestrally. It is 

 true our investigations have shown that some of the types of fresh- 

 water gill-bearing mollusca which existed in Mesozoic and Cenozoic 

 time are not represented among living forms, having become extinct; 

 but every family, almost every if not every genus, and many of the sub- 

 ordinate divisions of those genera that are known among living North 

 American fresh-water mollusca, have been recognized among the 

 species that constitute the different fauna?, the fossil remains of which 

 have been collected from the Mesozoic and Cenozoic strata of Western 

 North America, t These investigations also show the interesting fact 

 that while considerable numbers of types among the mollusca referred 

 to have been from time to time extinguished, the extinguishment hav- 

 ing in some instances taken place as late as the Pliocene epoch, few or 

 no new ones appear to have been introduced to replace any of them 



* These remarks are made with especial reference to the Unionidse ; but they are also 

 applicable to other gill-bearing mollusca, and they will no doubt apply with equal 

 torce to at least a part of the ichthyc fauna of that great river system. The progeni- 

 tors of the ganoids now living in that river system were doubtless originally land- 

 locked in the Laramie sea, continued through the fresh water Eocene lakes, and finally 

 escaped to the present river system in the manner already suggested. 



tThe extinctions referred to seem to have been caused by a failure of the waters in 

 which the lost types lived, to secure a continuous flow into any existing liver system, 

 This is of course equivalent to supposing an exception to the rule already announced, 

 that rivers have been persistent ; but such exceptions being well authenticated would 

 only add strength to the argument in favor of the rule. The portion of the Laramio 

 Group known as the Bear Eivcr beds, and the Miocene Truckee Group of Nevada, 

 Idaho, and Oregon, both containing extinct types, maybe taken sis indicating a failure 

 of the waters in which they were respectively deposited to secure persistent continuity 

 during subsequent time. 



