OP CONCHOLOGY. 129 



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. dissimilis in the 

 Atlantic, and Q-on. plicifera in the Pacific States. 



The very great influence which our two great chains ot 

 mountains has exercised, in restricting the distribution of our 

 species, may be inferred from what has already been said, and 

 requires no further allusion. 



The following observations on the geographical distribution 

 of the various genera and smaller groups, will exhibit some 

 very curious facts. 



10. 



Of this genus, the type of the Trypanostomoid form, there 

 are five species, two of which are smooth and three spinose; 

 they are of extremely localized distribution, being confined 

 to the head waters and tributaries of the Tennessee Eiver, 

 and principally to the Holston, in Southern West Virginia 

 and East Tennessee. They are very numerous in individuals, 

 as Mr. Anthony, during a visit made to this region several 

 years ago, selected and brought home several thousand speci- 

 mens. Prof. Haldeman also was very successful in collecting 

 them. 



PLEUROCERA. 



Of the eighty-four species, only thirteen are found so far 

 northward as the Ohio River, and only five of them originate 

 in that stream or its northern tributaries. The Tennessee 

 River and branches claims thirty-three species, of which 

 twenty-one appear to be confined to its waters. The Cumber- 

 land River contains four species identical with those of the 

 Tennessee, and about a dozen that are not found in the latter 

 stream. The Alabama River contains fourteen species, three 

 of which seem to be peculiar to it. These species are gene- 

 rally confined, however, to those portions of the Coosa and 

 branches that approach to East Tennessee. A few species 

 also inhabit the Tombigbee, of Mississippi. 



About a dozen species have the simple habitat " Tennessee" 

 stated; nine have "Alabama," and two "South Carolina." I 

 doubt very much whether the latter is correct. 



There is very good reason to believe that all the large 

 tuberculate, sulcate and angulate species inhabit the Tennessee 

 River, the most ponderous ones extending from the Coosa, 

 through middle and west Tennessee, to the Ohio River. 

 Among the angulate forms two, trivittatum and tortum, are 

 reported only from the Tombigbee and Chattahoochee Rivers 

 respectively. None of the carinate group — inhabitants of 

 Tennessee River — extend northward to the Ohio, but, strangely 



