xli 



States, a distribution coextensive -with our Vlvipa- 

 7^ Idee and other flimilies of Mollusca ; clearly indica- 

 ting that our country constitutes a distinct faunal 

 province. For, as the Vivijoaridce are replaced in 

 Mexico by AmpuUaria, so, for the Strepomatidce, 

 are substituted the more ponderous PachycliUi. 

 Between the former and the latter extend the broad 

 plains of Texas, with rivers devoid of species, 

 formino' a barrier to the interming^lino; of the two 

 groups. Besides this, the Mississippi River, from the 

 junction of the Ohio to its mouth, appears to have 

 formed a barrier to the westward progression of the 

 Strepomatidce, which but very few species have been 

 able to surmount. We believe that one species 

 only, — the Goniohasis sordlda, of Lea, — is common 

 to botJi sides of that great stream, while several 

 forms, all of Goniohasis, 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 the plausibility of the theory of a separate 

 creation of a small group of species, adapted to 

 w^ithstand 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 distribu- 

 tion, a striking parallelism with the distribution of 

 boreal marine Mollusca. Like the Unionidce, the 

 ViviparidcB, the Amnicolldce and the Limnctida', of 

 the same latitudes, the intercommunication afforded 



