INTRODUCTION. 



1\ 



of the Irish Lakes), Mons. E. Vincent (for Belgian fossil examples), 

 W. M. Webb, and C. E. Wright. Whilst the Zoological Museum 

 of Copenhagen furnished specimens of P. steenbucJiii (Moll.). 



In the lists of distribution the following abbreviations have been 

 employed for the names of these and other friends and helpers : 



