xlii 



by our waters has induced the plentiful distribution 

 of the same species from Iowa and Wisconsin to 

 Western New York, and even into Lake Champlain. 



AVe have already alluded to the total separation 

 of the species of our West Coast States. The 

 barrier of the Rocky Mountains has, of course, 

 jDroved with them even a greater obstacle than with 

 our IleUces. We find, accordingly, that the few 

 species (all Goniohases) mostly partake of two 

 common type characters, being either plicately 

 ribbed* or spirally striated. The Strei^oyyiatidce are 

 entirely absent from the waters of the New England 

 States, the exclusion being due probably not only to 

 the severe climate, for they inhabit streams in even 

 higher latitudes, but probably also their j^roximity 

 to the sea. There is no natural method by which 

 the species of the lakes could extend into the head 

 waters of the New England rivers, and none of the 

 species have as yet been transported by accident 

 across the intervening land. 



That the proximity of the sea exercises a great 

 disturbing influence on the very few species which 

 are exposed to and able to endure it, is proved by 

 the great mutations of form which characterize 

 Gon. Virginica and Anc. dissiinilis in the Atlantic, 

 and Gon. pliclfera in the Pacific States. 



The very great influence which our two great 

 chains of mountains has exercised, in restricting the 

 distribution of our species, may be inferred from 

 what has already Ijeen said, and requires no further 

 allusion. 



The following observations on the geographical 

 distribution of the various genera and smaller 

 groujDS, will exhibit some very curious facts. 



* Which strangely enough, equally characterizes a group of Gonio- 

 hases of East Tennessee. Our West Coast Helices are all of ditt'ereut 

 species and generally of quite distinct groups; Vivipara is excluded, 

 and the Amnicolidca belong to difiereut genera from those of the 

 Atlantic States, yet the same species of Phijsa, Lymnosa and Planorbis, 

 abound equally in either section ! 



