128 AMERICAN JOURNAL 



whicli but very few species have been able to surmount. We 

 believe that one species only, — the Ooniobasis sordida, of Lea, — 

 is common to both sides of that great stream, while several 

 forms, all of Gomobasis, are found inhabiting the western 

 tributary streams exclusively. 



Of course, our great river does not interpose such a 

 formidable barrier in the northwest, where its volume is much 

 less, and we here find the species of the great lakes not only 

 inhabiting its waters in abundance, but extending into its 

 western branches. 



The species of the great lakes, though few in number and 

 small in size, are very numerous in individuals, yet they fade 

 out as completely on approaching the Ohio Eiver as do the 

 southern species; we are, therefore, compelled to admit in this 

 case tho plausibility of the theory of a separate creation of a 

 small group of species, adapted to withstand the rigors of 

 a climate which effectually forbids the introduction of the 

 meridional species. 



We may discover in the paucity of species, their small size 

 and scant ornamentation, but multiplicity of individuals, and in 

 their very extended distribution, a striking parallelism Avith 

 the distribution of boreal marine Mollusca. Like the 

 Unionidse, the Viviparidse, the AmnicoUdse and the Limnseklse, 

 of the same latidudes, the inter-communication afforded 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. 



We have alreadj'- alluded to the total separation of the 

 species of our West Coast States. The barrier of the Eocky 

 Mountains has, of course, proved with them even a greater 

 obstacle than with our Helices. We find, accordingly, that the 

 few species (all Goniobases) niostl}^ partake of two common 

 type characters, being either plicately ribbed* or spirally 

 striated. The Sirepomatidw are entirely absent from the 

 waters of tlie 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 to their proximity 

 to the sea. There is no nattiral 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 proximitj^ of the sea exercises a great disturbing 



* Which, straiigelj'^ enough, equally characterizes a group of Ooiiiobases 

 of East Tennessee. Our West Coast Helices are all of different species 

 and generally of quite distinct groups ; Vioipara is excluded, and the 

 AmnicolidcB belong to ditferent genera from those of the Atlantic States, 

 yet the same species of i'hysa, Lymncua and Planorbis^ abound equally in 

 either section ! 



