OF NORTH AMERICA. XI 



Chapter X includes the bibliography, which is comparatively ex- 

 haustive, although a few references have doubtless been overlooked. 

 It is believed to contain all of the important references to the subject. 



Of the 211 names occurring in the literature, 95 are herein ad- 

 mitted as worthy of recognition as species or races. 6 names are 

 used for the first time. 35 fossil species have been recorded, of 

 which 24 are here recognized. One new race is described. The 

 absence of keys for the determination of the species may lead some 

 readers to suppose that the author places no value on aids of this 

 character. Such is not the case, and many attempts have been made to 

 prepare a satisfactory key to aid beginners, but without success. In a 

 group like the Lymnaeas, where variation is paralleled in so many of 

 the species, it is obviously impossible to prepare a key which will be 

 intelligible to any save those well versed in the subject, and for this 

 class no key is necessary. It is believed that with the figures and de- 

 scriptions no one will have serious difficulty in determining the ma- 

 jority of the species of American Lymnseas. 



It is fully realized, in presenting this monograph, that there is 

 much contained therein which is faulty and incomplete. In a few 

 cases adequate material has not been available for a satisfactory treat- 

 ment of the species. There are many records which have not been 

 substantiated and have been omitted, therefore, from the records and 

 distribution maps. In a number of cases the soft parts have been lack- 

 ng and a definite placing of the species has been impossible. With all 

 its shortcomings, however, the author believes that it is a step in ad- 

 vance of previous efforts, and that it will prove an aid to all students 

 of this interesting family. 



Should this volume meet with the approval of the conchological 

 public, it is in the mind of the author to prepare companion volumes- 

 on Planorbis, Physa and related genera. 



FRANK COLLINS BAKER. 

 Chicago, Illinois, January, 1911. 



