PREFACE. 



The fresh-water snails embraced in the family Lymnseidse form 

 an important factor in the mollusk fauna of the more northern coun- 

 tries of the world. Say, Haldeman, Lea, Tryon, Binney and other early 

 writers have described many species and several of these authors have 

 essayed more or less complete monographs of the family. In 1865, 

 W. G. Binney monographed the Lymnseas in the series of illustrated 

 papers published in the Smithsonian Miscellaneous Collections under 

 the litle of "Land and Fresh-Water Shells of North America." Bin- 

 ney was naturally very conservative and reduced the then known spec- 

 ies and varieties (79) to about one-third this number (31). Though 

 expressing his opinion that certain species were synonymous with 

 certain other species, he was careful to include descriptions and figures 

 of all known forms, leaving the matter to the judgment of American 

 conchologists to weigh and test his conclusions. Unfortunately the 

 American conchologists did not do this, but accepted Binney's con- 

 clusions as final, thus greatly complicating the literature and rendering 

 the use of the latter for geographic purposes almost impossible. 



The present monograph was begun as a revision of Binney's work ; 

 but, as the matter progressed, it became evident that the old classifi- 

 cations and methods of presentation needed an almost complete revo- 

 lution to meet the requirements of modern standards. The meager 

 descriptions needed to be much more extensive, the exact geographic 

 and geologic distribution needed to be indicated, the ecology as well as 

 other items demanded attention, and the project has thus grown to 

 many times its original dimensions. It was also evident that to ade- 

 quately present the subject a large number of figures would be neces- 

 sary and these have accordingly been made from authentic or type 

 material. It is obvious that until a group in any branch of Zoology is 

 placed upon a sound basis and the limits of the species accurately de- 

 fined, no discussions of zoogeographic questions can be of any value. 

 This fact has been clearly demonstrated in the course of the present 

 work. 



It is the purpose of this volume to furnish to students of this 

 family of fresh-water mollusks a reliable guide to a knowledge of 

 the species, of their habitat relations and their geographic range. To 

 this end chapters more or less extensive, have been introduced briefly 



