Vlll 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 1 833 ; the latter paper is reprinted in Dr. 

 George Johnston's Introduction to Conchology. 



