^{page 1 note *} Not having had the opportunity of consulting many of the original figures of the French naturalists, I have to acknowledge the great advantage I have derived from the study of Mrs Gray's excellent etchings, in helping me to determine species, as also the descriptive letterpress of Dr Gray. Many of the anatomical particulars detailed by me had, of course, been previously observed by others; and though I have not clogged the paper with the separate announcements, dates, and authorities of every addition to our knowledge of Heteropoda,—which, indeed, would be no small task,—I can vouch for it, that all the facts embodied in the text have fallen under my own observation, and they may therefore be regarded either in one sense as original matter, or in another, as confirmation of what had been already made known.

^{page 2 note *} I have made use of the convenient terms Gymnosomata and Thecosomata in a similar sense to that in which De Blainville applied them to the Pteropoda.

^{page 15 note *} All the species of Atlanta present so close a general resemblance, that their nice discrimination requires careful examination and comparison. The keel may be more or less, or not at all interposed between the peristome and the body whorl. It may be plain or wavy, though the shell never exhibits this latter character, as it does in Carinaria. The spire, as to its prominence, depression, evenness or obliquity, closeness or openness of its whorls, smoothness, dotted surface, or linear markings, affords us characters which do not appear to have been hitherto sufficiently recognised. There is, moreover, in colour, which one would naturally regard as being of little importance, a scarcely ever failing peculiarity of species.