Geographical Distribution of Certain Fresh-Water Mollusks. 319. 



States, and though wanting across the whole interior of the continent, 

 re-appears in the drainage of the Pacific slope. Westward, as far as 

 the State of New York, we have a few species added to the New 

 England list of TJnionidm^ and a new faunal factor in the presence of 

 four species, and three genera of the Strepomatidoi. It is worthy of 

 remark, that, as the present understanding of these shells goes, two 

 of these species of 8trepomatid(B are widely distributed over the 

 northern States to the west, while the other two have their range to the 

 south, along the western slope of the Appalachians, to Pennsylvania, 

 Virginia, etc. The same fact is to be observed with regard to the 

 Unionidoe of New York, several of the species belonging especially to 

 the Appalachian drainage, while others have a wide western and 

 southern range. 



We may now direct attention to the Ohio drainage. The number of 

 described species of Unionidm, from North American localities, in 

 1874, including those in Mr. Lea's vol. xiii., was 832, of which 82 were 

 described from the Ohio river, exclusive of 10 or 15 species from the 

 Scioto, Wabash, and other northern tributaries. Of these 70 were 

 Uniones, 7 Ilargaritanas and 5 Anodontas. It is thus seen that one 

 tenth of all species described have been from Ohio river types, and in 

 very many of these cases the words " at Cincinnati" are added. When 

 the wide system of drainage from which our river receives its waters 

 is taken into account, a region embracing the whole western slope of 

 the Appalachians, from southern New York to the northern part of 

 Georgia, and including not only wide climatic variation, but an infinity 

 of other conditions, depending upon previous geological causes, it will 

 not be out of the province of facts to say that here we have the most 

 important field for stud3^ Now, what are the phenomena which it 

 presents ? Pushing our inquiries westward, across the States of Indi- 

 ana, Illinois, etc., to the base of the Rock}^ Mountain plateau, we find 

 that the fauna is essentially that of the Ohio river at Cincinnati. 

 There are a few species (?) interpolated across this region, and in the 

 Wabash two forms of StrepomatidcB at least are found which belong to 

 the Southern part of the Ohio drainage, and really to the mountain- 

 ous portion of it. Across this western range we find the shells to be 

 very much varied in weight, size, nacreous color, outer marking, and 

 perfection of form, it being as rare to find an " eroded" shell in the 

 Wabash, White river or Sangamon, as to find a perfect one in the Ohio. 

 Into the literature of these shells has crept a large syuou^mi}', which 

 reaches both the families under consideration, and is the result of 



