PREFACE. V 



To Mr. John Ponsonby, of London, thanks are due for numerous 

 rare or new species of Helices, many of which have been figured in 

 the Manual, and more especially for the correction of errors in 

 synonymy, localities, etc., occurring in previous volumes of this 

 work. Mr. G. K. Gude has rendered me a similar service ; and 

 from a very large number of conchologists both in America and 

 abroad, I have received information upon particular species and 

 genera, for all of which I would here express my gratitude. 



Summary. In this volume the author has essayed to indicate the 

 primary groups of the Helicidae, arranging the genera accord- 

 ing to a few main types of internal structure, in place of the chaotic 

 or arbitrary sequence of groups hitherto prevailing. The multitude 

 of groups recognized are shown to be reducible to about fifty genera 

 distinguished by structural features of importance, which are de- 

 scribed and illustrated, lists of the living species of each genus being 

 given. An outline of the distribution of the main groups is offered, 

 with hypotheses of the probable migrations and phylogeny of these 

 groups. Incidentally, the comparative value of the genitalia, shell, 

 jaw and radula in classification, and the laws of their modification 

 are worked out in some detail. Finally, the nomenclature of Helices 

 has been thoroughly revised, and, it is hoped, placed upon a sound 

 basis. 



It rests with the critical and discriminating conchological public 

 to decide whether the author of this volume shall undertake a com- 

 panion work on the genera of Zonitidse and Agnatha. 



H. A. P. 



