288 ORTMANN— THE ALLEGHENIAN DIVIDE. [April 18, 



of Simpson, 1893, p. 354, and 1900a, p. 505, pi. 18). But it should 

 be noted, from the beginning, that this holds good only for certain 

 groups of animals, while in others no such differentiation is observed. 



While this appears to be correct in a general way, investigations 

 on the details of the relations of the two faunas on the eastern and 

 the western side of the Alleghenies are very few. In fact, there are 

 none whatever that have treated this question from a broader view- 

 point. The most elementary requirement, the study of the actual dis- 

 tributional facts of freshwater animals, had been greatly neglected. 

 From most of the more important rivers (Susquehanna, Potomac, 

 Allegheny, Monongahela, Kanawha) hardly any observations were 

 at hand, which would have permitted any definite opinion as to the 

 general character of their faunas, and in the region of the head- 

 waters of these, our previous knowledge was a blank. 



For this reason, the present writer had first of all to undertake 

 the task of obtaining reliable and complete data with regard to the 

 fauna of the various streams running off the Alleghenian divide. 

 In the course of these studies it became evident that the most im- 

 portant group of freshwater life is formed by the A^ajades or Fresh- 

 zvater Mussels. They offer two advantages : first they are very rich 

 in species, the natural affinities of which are now rather clear ; and 

 second, they are forms which apparently possess no exceptional 

 means of dispersal, that is to say, they are, as a rule, unable to cross 

 from one drainage system into another over land (either actively or 

 passively). This opinion of mine agrees with that held by Simpson 

 (1900b), but is in sharp contrast to that expressed by Johnson 

 (1905), who believes that "shells" or " mollusks " in general, and 

 also especially Najades, have frequently been dispersed by birds, etc. 

 Such cases may happen among the Najades, but they cannot be con- 

 sidered as the normal way, and Johnson's view rests upon very 

 inadequate ideas about Naj ad-distribution, and chiefly the instances 

 of apparent discontinuous distribution of species, which would favor 

 the assumption of transport, are, without exception, founded upon 

 defective facts. (It should be remembered that the chief means of 

 dispersal of the Najades consists of transport in the larval state by 

 fishes, on which the larvse are attached ; but this precludes the possi- 

 bility of transport over land.) 



