PREFACE.
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 repetitions
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.