104 
INTRODUCTORY REMARKS. 
daily for nearly a month in examining the animals of the various species of 
Mollusca which the Baron Cuvier and his pupils had been collecting together 
for several years, and which he had most carefully preserved for his own use, 
for his intended new edition of his anatomy of Mollusca, a work the loss of 
which we must all most sincerely regret. 
All the descriptions (with the exception of a few of the English species) 
are taken from animals preserved in spirits, and which had in consequence 
became much contracted in many of their parts. 
I have only given a zoological description of the animals ; as according to 
the rules of the British Museum, we are very properly forbidden to dissect 
the animals under our charge, which might thereby be rendered useless for 
subsequent observers. Sensible of the propriety of this regulation, I was 
careful not to injure any of those which I examined in the French and Dutch 
museums, although in many cases there were several duplicates, the ana- 
tomical details of which may hereafter be examined by those who are able 
to bestow more time on the snbject. 
The examination of the numerous genera of Mollusca which I have been 
enabled to make, has not only induced me to alter the position of some of 
the genera, but has also discovered to me the important fact, that there is 
often a great difference between the animals of two very similar shells. This 
is very apparent between Littorina and Assaminea, the shells of which 
resemble each other so nearly as scarcely to be distinguishable, and yet the 
animals are very different. 
These descriptions, short as many of them are, will be regarded as of 
more importance to the Zoologist, when it is considered, that, with the 
exception of the few animals dissected by Cuvier, and published in the 
Annales du Museum ; and those which Blainville examined in the collection 
