vi PREFACE. 
words are required in order to indicate in what respects this edi- 
tion differs from its predecessor. In the first edition the work 
consisted of three parts, in this it consists of two. In Part I. 
is comprised the general remarks on the structure, distribution, 
&e., of the Mollusca, while Part II. is devoted to the Sy- 
nopsis of the Genera. The chapter on Tunicata has been 
omitted, since they are more nearly allied to the Polyzoa 
than to the Mollusca proper, and since the treatment of the 
Molluscoidan group would have made the work inconveniently 
bulky. It seemed preferable, therefore, to devote a future 
volume of the series to the Molluscoida (embracing both the 
Tunicata and the Polyzoa) than to describe them in the 
present work. The book has been subjected to a complete 
reyisal, and numerous alterations and additions have been 
made; but the reviser has interfered as little as possible 
with the Author’s original classification and systematic 
arrangement. 
A. B. 
Sept., 1866. 
