Vlli 
PREFACE. 
The description of the species is preceded by a 
general outline of the distribution of molluscous 
animals ; and the genera have been distributed into 
natural families from characters taken from the con- 
sideration of the animal^ which alone can be regarded 
as the proper subject for classification. Every day 
proves, to the scientific conchologist, that every 
modification in the structure of the animal impresses 
its character on the shell, and that the shells thus 
afford good subsidiary characters for the distinction 
of groups. 
In the observations appended to the characters of 
groups and species, there is added, as opportunities 
occurred, a physiological account of the mode of for- 
mation and structure of the different parts of shells.* 
It is to this part of the subject that I would especially 
direct the attention of my readers ; as, in the study 
of the structure, formation, development, and colour- 
ing of the shells, and in the habits of the animals 
which form them, they will find a never-failing source 
of pleasure and instruction, which can be carried on 
without any expense. For these particulars are as 
easily to be observed in the most common snail, as in 
the finest and most expensive shells in the cabinets of 
For a larger exposition of my views on this subject, refe- 
rence should be made to my papers published in the first 
volume of the Zoological Journal^ and in the Philosophical 
Transactions for 1833; the latter paper is reprinted in Dr, 
George Johnston’s Introduction to Conchology, 
