The singular object now before us, a shell no less remarkable for the peculiarity of its form than rarity of occurrence, is one of the most choice productions of the seas surrounding the Friendly Isles. The discovery of this shell, like that of many others, resulted from the assiduities of that eminent Naturalist and promoter of scientific knowledge, the late Sir Joseph Banks, and of Dr. Solander, who accompanied him in that memorable voyage of Captain Cook to the Southern Hemisphere, in which the Friendly Isles were discovered. The fine example of this shell, in particular, from which the drawing in our plate is taken, it may be also added, was one of those which were brought to this country by Captain Cook upon the return of the expedition, and which being shortly after presented to Sir Ashton Lever, remained in the Museum of that distinguished amateur from that period to the time of its dissolution in the year 1806. When we consider the very remote situation of those islands, so distant from the usual track of all navigators, we cannot be surprised, admitting the species to be local in those seas, to find it has remained a very rare shell from the period of its discovery to the present time. In the course of many years only a few specimens have occurred to our observation, and while it has remained scarce with us, it appears to have been still more uncommon in the continental cabinets: very few of which, if we are informed correctly, were lately in possession of it. The first difficulty that arises in the mind of the naturalist upon the inspection of this shell results from the ambiguity of its generical peculiarities: we pause to consider where it should be placed. Linnæus, to whom, as it will be observed, the present shell was totally unknown, arranged the Hammer Shell, its nearest approximation, among the Ostreæ. The Hammer Shell, or as it is more usually denominated the Hammer Oyster Shell, had been discovered before the time of Linnæus; it had appeared in the work of Rumpfius, Seba, Gualtieri and Argenville, and the shell had been examined and described by him in the Museum of Ulrica, Queen of Sweden, under the name of Ostrea Malleus. That the hinge accords in some degree with that of the Ostreæ generally must be admitted, at the same time that it possesses other characters less easily reconciled to that genus, unless we embrace the Linnæan genus in all its latitude, and to this the conchologist of the present day cannot accede, at least without some little difficulties.