19I3-] ORTMAXX— THE ALLEGHEXIAX DIVIDE. 381 



SUMMARY OF CONCLUSIONS. 



i. I think that the present studies have demonstrated the funda- 

 mental fact, that certain freshwater animals are apt to furnish im- 

 portant evidence for past conditions of drainage by their present 

 distribution, while others are not. The most important of the 

 former are the X a jades. There are many cases (not only in our 

 region) where indentical or closely allied species are found in dif- 

 ferent drainage systems which have at present no direct water con- 

 nection. Such cases are generally restricted to limited, well-defined 

 regions. 



In our region we have seen that such cases exist in the mountains 

 in the section which has the upper New River for its center; but 

 similar instances are known in Pennsylvania, in the headwaters of 

 the Susquehanna. 



This localisation is the most important evidence against the 

 assumption that passive transport over land has played a part in 

 these cases : if this was possible at all, or if it was a factor to be 

 considered, evidence for this should be general. But just where 

 we might expect that transport should have worked by all means, 

 there is no evidence whatever for it. This is most especially true 

 in the case of the divide between the upper Tennessee drainage and 

 that of Xew River. If Najades should be able to cross divides by 

 being transported, it should have happened just here. Also the gen- 

 eral condition of the eastern and western fauna, its dissimilarity, 

 shows that X a jades were not transported across the mountains. 



Very likely the freshwater snails of the family Pleuroceridce 

 submit to the same general law as the Na jades and are important for 

 the study of the old drainage features. But they should be further 

 studied, chiefly with regard to their actual distribution, their sys- 

 tematics and relationshios. Finally, some crayfishes of the genus 

 Cambarus are extremely valuable in this respect, but unfortunately 

 their number is not great. 



2. The Allegheny system forms an old and very well-marked 

 boundary between aquatic animals inhabiting the interior basin and 

 the Atlantic slope. This barrier may have been rendered insignifi- 



