VI PEEFACE. 



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, 

 &c., of the MoUusca, 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 MoUusca proper, and since the treatment of the 

 MoUuscoidan 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 

 revisal, 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. 



