PEEFACE. 



A much longer time has elapsed in the publication of this Monograph than was 

 originally anticipated. When the first Part was issued, ten years ago, the study of the 

 Nudibranchiate Mollusca was comparatively in its infancy, and the extent to which the work 

 might reach was, therefore, pretty much a matter of conjecture. Few branches of science 

 have advanced more rapidly than this has done during the period. New views have opened 

 up, and materials have accumulated in our hands to such an extent that it has been found 

 impossible to keep the work within the limits originally prescribed. Should, therefore, any 

 want of unity be detected in its execution, it must be attributed to this cause. Some repeti- 

 tions have been found unavoidable, arising partly from the irregular mode of its issue, and 

 partly from the desire to give the information we possessed while it was yet new. Our 

 opinions, too, have occasionally undergone a little modification as the work advanced.* We 

 trust, however, that the disadvantages arising from these causes will not be found of any 

 great importance, and that the increased information we are enabled to give will be thought 

 a sufficient compensation for the delay. 



The valuable assistance we have received from our friends during the progress of the 

 work claims our most grateful thanks. Every new species found by other naturalists has 

 been unreservedly communicated to us as it occurred, and manuscripts, drawings, and 

 specimens relating to them, or illustrating species already published, have been liberally 

 placed in our hands. For assistance of this kind we are especially indebted to our late 

 lamented friends, Professor Edward Forbes, and William Thompson, Esq., of Belfast. To the 

 Rev. Dr. Fleming our thanks are due for the like assistance. Nor should we omit to mention 

 the instruction we have gained from the specimens and drawings of Scandinavian species 

 intrusted to us for examination by Professor Loven, of Stockholm ; and the numerous fine 



* To account for any discrepancy that may thus occur, we have given a list of the contents of each 

 Part, with the dates of their publication. 



