Xi INTRODUCTION. 
ideas, in attempting to arrange the British species of the 
Mollusca into classes, orders, families, stirpes, genera and 
species, by giving an enumeration, with the descriptions, of 
all the British species that have absolutely come under my 
individual inspection. 
I feel that I shall be accused (as I was when I first sub- 
mitted my arrangement of the Crustacea, Arachnoidea, Acari 
and Insecta to the public eye) of having constituted too 
many genera. It is necessary for my justification to observe, 
that although I formed families containmg but one known 
genus, and genera with but one species, many of these va- 
cancies have been filled up by Cuvier in his ‘Regne Animal ;? 
by Rafinesque, Say and other authors. I shall therefore 
in this Work follow precisely the same plan which I for- 
merly did in the above-mentioned classes. 
Respecting the names that I have given to what I con- 
sider distinct genera, I have always invariably named the 
genera, as far as possible, from their essential characters ; 
except only when I have perceived that the names of the 
parts constitutmg a generic distinction might probably 
equally apply to some other genus not yet discovered ; and 
where I have not been enabled to find sufficient and certain 
essential characters, I have followed the rule laid down by 
Fabricius, the first naturalist who attempted to form a 
natural arrangement of Insects,—-‘‘ Nomina generica nil 
significantia omnino optima ;” and as far as possible I have 
selected, according to the rule laid down by the same 
author, that “Nomina barbara nullo modo sunt toleranda.” 
I have adopted the following Classes ; the Cephalopoda, 
Pteropoda, Gasteropoda and Brachiopoda of Cuvier; the 
Ascidiz of Savigny; and the Acephala of De Lamarck and 
Cuvier: in this last Class, the Acephala, I have taken the 
liberty of changing its name and substituting another, 
which I think more consistent, Ginglymaconcha, signifying 
shells having a hinge; because, although they have no 
