52 MEMOIRS NATIONAL ACADEMY OF SCIENCES. [vol.xii. 



lakes, and the river systems of which they constituted lacustrine portions. The Ohio and Upper Mississippi, the two 

 most ancient portions of the present great system, were once separate rivers, emptying into a northern extension of the 

 Great Gulf; and it is practically certain that neither of them received that portion of the molluscan 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. 



He further remarks that this last statement applies particularly to the UnionidaB, but is 

 also applicable to the giU-bearing mollusca and some fishes. Later White ('05) reiterated this 

 opinion regarding the origin of the Unionidse. Simpson ('96, p. 336) accepts this view also. 

 I know of no dissenting opinion from this position. 



On the other hand, something may be said in favor of the view that they originated in 

 southeastern United States, and that their spread was westward, in a manner similar to the 

 post-Glacial migrations of life into the glaciated region. The following facts and inferences are 

 favorable to this alternative hypothesis : 



Southeastern United States (exclusive of the Costal Plain) has been a land aiea sufficiently 

 long, since the close of the Palaeozoic, to have been the original home of the Laramie types of 

 molluscs. The southeast was being base-leveled while the Laramie strata were being formed. 

 At this time the southeastern streams were well developed, particularly certain ancient streams 

 or drainage lines, such as the New-Kanawha, upper Tennessee, and Cumberland drainage, 

 Coosa-Alabama and the upper French Broad. It seems very improbable that these streams, 

 so favorably located with regard to lime-bearmg rocks, favorable temperature, abundant and 

 suitable food, clear and rapid water, and abundant rainfall, should have remained unpopulated 

 by shells showing a preference for rapid water from the close of the Palaeozoic, and had therefore 

 developed no endemic element which can now be recognized. If such a fauna was present, 

 some fragments of it at least are to be expected mixed with the derived western element. 

 Such an element has not been recognized or even been suggested. Of coiirse if this region 

 was whoUy stocked from the West this native element would not be expected. Had the south- 

 eastern streams been fvilly stocked their population might have retarded or prevented the 

 arrival of further invaders. The southeastern streams have been favorable for a stream fauna 

 so long that it seems almost gratuitous to assume that the mollusca have come from elsewhere, 

 rather than that they have had a continuous development where they now flourish. The 

 diversity we now find in these animals is such as might be expected in a group long resident in 

 a region. 



The present endemic element in the southeastern streams is so large, there being hundreds 

 of species peculiar to the region, and a large number are confined solely to certain streams or 

 systems. This has the appearance of vicarious endemism rather than relict endemism, because 

 in so many cases the intei gradations between the forms now exist in very large numbers, and 

 these would probably be lacking in the case of relict endemism. 



The lack of fossils in the Southeast may be urged against this view. This old land mass 

 has been one mainly occupied by streams rather than lakes and other water bodies, and stream 

 deposits are very rarely preserved, particularly in a region subjected to prolonged erosion and 

 repeated base-leveling. The paucity of streams deposits is recognized in the following manner 

 by White ('85, p. 484) : 



The discovery of so few traces of fluviatile deposits as have been made among the strata of the earth is probably due 

 to the persistent adherence of rivers to their ancient channels. * * * If the land continued to rise, as has been so 

 generally the case in the gradual production of the North American continent, the earlier river deposits were swept away 

 in later times by their waters, as their valleys were broadened and deepened. It is therefore, as a rule, only in deposits 

 of the lacustrine portions of ancient river systems that their faunae have been preserved. 



Although the Pleurocerid genus Goniobasis was abundant in the Laramie, White ('85, p. 

 465) knew of none found in deposits later than the Eocene, although the genus has lived here 

 continuously and abounds today in the Southeast. This lack of fossils in the presence of the 

 evidence of the surviving animals shows how little weight the lack of fossils carries in the present 

 case. 



Thus in spite of the older fossil remains in the West, the physical and biotic histor}' of the 

 southern Appalachian region and the present apparent vicarious endemism in the family 



