Geographical Distribution of Certain Fresh-Water JfoUusJcs. 323 



ences that in the slightest degree obscure their character, or give rise 

 to a suspicion of new species. Of the Ohio types that are thus specifi- 

 cally persistent, may be mentioned the U. tuherculatus, TJ. cylindri- 

 ctis, U. irroratus, U. anodontoides, JJ. cornutus, U. rectus, U. trian- 

 gularis, Marg. dehiscens, etc. 



This is not a statement that these shells have no varieties, or local 

 differences; but it is a fact that none of these have ever presented any 

 varieties that would lead to the suspicion of species. As these are all 

 shells belonging to the group of Ohio types, this truth in the light of 

 additional ones, may prove to be a factor of considerable consequence. 

 As opposed to this fact, may be mentioned the opposite one, equally 

 well attested, that many of the Ohio types present a very great range 

 of variability, and have given rise to an immense synonym}^ 



We have now before us a series of facts which may be briefly 

 epitomized as follows: 



First— The small number of species of UnionidcB, and the entire 

 absence of StrepomatidcB in the New England States, and the fact of 

 the distribution of some of the former entirely across the continent to 

 the Pacific coast, and southward along the Atlantic. 



Second — The introduction of the StrepomatidcB, west of the Green 

 Mountain uplift, and their division into two geographical groups, one 

 pertaining to the western, and the other to the southern fauna. 



Third — The continuance of the Ohio types of Unionidce westward, 

 north of that stream, to the limits of the Mississippi drainage, and 

 south and southwestward to western Texas, and the comparative ab- 

 sence of the Strepomatidce over this area. 



Fourth — The introduction of new species in both families, and of 

 new genera in the Strepomatidoe^ so soon as we cross the Ohio and 

 travel south. 



Fifth — T\\Q fades of the groups of species which the streams of this 

 part of the Ohio drainage contain, stamping them as diff'erent faunas. 



Sixth — The anomalous fauna of the Alabama drainage, and espe- 

 cially the fact of its geographical isolation. 



Seventh-^T\iQ special cases of the only species of spinous Unio 

 known, and that of the M. marg aritif era. 



Eighth — The persistent specific character of some mollusks, and the 

 excessive evidence of variation in others. 



In looking over this summar}^, as sustained by large collections and 

 extensive experience in collecting, an experience that included geolo- 

 gical examinations of all the districts, it appears certain that other 



