﻿iv INTRODUCTION 



the purpose of this publication will have been fully accom- 

 ])lished. 



While there is no doubt but that the increase of our g-eneral 

 knowledge of the anatomy and embryology of the family will 

 lead to and necessitate many changes in the arrangement pro- 

 posed in the Synopsis, a large part of the more recent work 

 that has been done is still in a tentative and fonnative con- 

 dition and will be subject to more or less modification in 

 the future, and many important groups still await the compre- 

 hensive examination that will be required to definitely deter- 

 mine their systematic position. As this work is intended to be 

 a companion volume to the Synopsis and is especially intended 

 to facilitate the study of the species, it has been deemed best 

 to retain the systematic portion substantially as given in that 

 work. 



In order to bring tlie subject down to date. I have inserted 

 in their proper places the original descriptions of all the North 

 American species that have been published since the Synopsis 

 appeared and also those of such foreign species as have been 

 brought to my attention. But my long continued residence in 

 the south, away from the large museums and without library- 

 facilities, has prevented me from expressing any personal opin- 

 ion as to their validity and they are published without critical 

 examination and simply for what they may be worth. As far 

 as possible I have inserted them in what appears to be their 

 proper systematic position, but in far too many cases, the re- 

 prehensible habit of many authors of describing supposed new 

 species without any comparative remarks as to their relation 

 to those already described and of ignoring all the recent ad- 

 vances in classification has compelled their reference to the 

 class — "mcertiv scdis." 



I have not attempted to include the perennial vagaries of 

 the Continental conchologists in dealing with the European 

 fauna. T have already had occasion to express my opinion of 

 their work in the Synopsis and have seen no reason to change it. 



Since the publication of the Synopsis, it appears to have 

 been demonstrated that in the ircnus Parrcxsia the embrvos 



