The curious floral forms of the Zoöphytes have, as yet, attracted the greatest degree of attention among the constructors of marine Aquaria; yet other forms of ocean life offer equal, if not superior, opportunities for curious observation, and are equally well calculated to bear the confinement of the tank. Among them all, none more than the Molluscs, especially the shell-bearing division, which merits the careful attention of the student of Nature, as forming some of her most singular manifestations. The knowledge of the nature and structure of many of the most curious shell-bearing Molluscs is of very recent date, with the exception of those which possess obvious qualities which have fitted them for articles of diet or commerce; such as the Oyster, Mussel, Cockle, &c., as eatable species—and the Purples, Sepias, and Cuttle-fish, as containing valuable dyes. With the exception of such as these, the pearl-yielding Bivalves, and a few others, nothing was known, with a few remarkable exceptions, of the animals that create and inhabit the beautiful shells that have so long been ranked among the most elegant objects of the cabinets of the curious. Many of these were, in fact, scientifically classified by learned naturalists before the nature of the animal, of which they formed the mere senseless husk, was even guessed at. The ordinary collector did not even desire to know anything of the creature which produced the shell he most prized; it was sufficient for him that it was estimated as “rare” by his brother collectors—rarity being a quality more highly prized than even beauty. With this feeling, prices as great were given for single shells as ever enthusiastic Hollander paid for a coveted bulb during the height of the Tulipomania. No amount of guineas was too much, at a sale of shells, for such a contested prize as a _Many-ribbed harp_, a _Gloria maris_, a _Cedo nulli_, or a _Voluta Junonia_. But that race of idle shell-fanciers has given place to a race of true conchologists, who are investigators as well as collectors, and whose labours are daily developing unexpected and valuable knowledge from those long obscure pages of the great book of Nature. The marine Aquarium may be made the means of many curious discoveries regarding the habits and organization of the shell-bearing Molluscs; and, with this feeling, I may direct the attention to several of their singular characteristics, in order that they may serve as clues to the detection of others.