PREFACE. 
IT has long been the fashion to present the reader with 
a prologue ; which, however, I have observed, of late 
years, has become shorter and shorter. The utility of 
this practice is questionable, as the body of the work 
includes all that can be said in a preface, and by it we 
forestall the pleasures of expectation; besides, be it 
good or bad—and the author is frequently not the 
writer—it often unjustly decides the fate of a book, 
causing a bad one to be read, or a good one to be 
thrown aside; nevertheless I have in some measure 
complied with an old custom, by giving a few prelimi- 
nary remarks in the introduction to the classification. 
It will be observed that some of the families and 
genera of the Cephalopoda and Pulmonifera have been 
admitted into the sketch of the classification to com- 
plete the chain of the Molluscan series; to enter on 
their consideration forms no part of the plan of this 
work, which relates exclusively to the Brztish Marie 
‘l'estaceous Mollusca. 
In some instances “ nobis”’ is attached to the name 
of well-known species ; this has been done, not from 
any desire to assume credit for the labours and disco- 
