19.3.] ORTMANN— THE ALLEGHENIAN DIVIDE. 327 



It is not necessary to give a detailed account of the single species ; 

 it suffices to enumerate those species which I have before me from 

 both sides of the mountains : 



Sphccrium sulcatum (Lam.) 

 SpJiccriuin solidiilum (Pr.) 

 Sphccrium stamineum (Conr.) 

 Sphcerium striatinum (Lam.) 

 Musculium transversum (Say) 

 Miisculium tntncatmn (Linsl.) 

 Pisidium virginicum (Gmel.) 

 Pisidium compressum Pr. 



Of course, these examples will become more numerous when 

 more exhausting studies have been made. 



Altogether, we may safely assume that it is a general rule among 

 this group, that the distribution is not influenced by the Alleghenian 

 divide. As we have seen above, this condition is extremely rare 

 among the Najades. In the present case, the distribution of the 

 Sphccriidcc seems to have been formed under the influence of one 

 great general factor, which probably is the faculty of these shells to 

 cross over divides, presumably by being transported. It is very 

 pertinent to bring this out here most emphatically, because, as we 

 have seen, this factor has had very little or no effect among the 

 Najades, as is shown by the entirely different character of their 

 distribution. 



II. GASTROPODA, FAMILY: PLEUROCERID^. 



The identifications have been kindly furnished by A. A. Hinkley. 

 I have a rather satisfactory material of this family, although the 

 records are not as complete and exhausting as in the Najades. 



The whole character of the distribution of these freshwater snails 

 is like that of the Najades, and, consequently, it is indicated that no 

 exceptional means of dispersal (transport) have played a part. The 

 range of the species follows rather closely the river systems, and 

 the effect of the Alleghenian divide as a barrier is quite evident. 

 Two facts, however, are to be regretted, first, that in the region 



