There are many features connected with the internal structure and surface of the shells of molluscs that are quite as interesting and instructive as those exhibited externally; hence a collection of the shells intended for future study should display internal as well as external characteristics. Thus, some of the spiral univalve shells may be ground down on an ordinary grindstone in order to display the central pillar (the _columella_) and the winding cavity that surrounds it, while others, such as the cowries, may be ground transversely to show the widely different character of the interior. Bivalve shells, too, may be arranged with the valves wide open for the study of the pearly layer, the lines of growth, the scars which mark the positions of the muscles that were attached to the shell, and the teeth which are so wonderfully formed in some species. Some collectors make it a rule to thoroughly clean all the shells in their collection, but this, we think, is a great mistake; for when this is done many of the specimens display an aspect that is but seldom observed in nature. Many shells, and especially those usually obtained in deep water, are almost always covered with various forms of both animal and vegetable growth, and it is advisable to display these in a collection, not only because they determine the general natural appearance, but also because these growths are in themselves very interesting objects. Further, it is a most interesting study to inquire into the possible advantages of these external growths to the inhabitants of the shells, and _vice versâ_--a study to which we shall refer again in certain chapters devoted to the description of the animals concerned. But there is no reason whatever why some of the _duplicate_ specimens should not be cleaned by means of a suitable brush, with or without the use of dilute hydrochloric acid (spirits of salt), or even polished, in some few cases, to show the beautiful colours so often exhibited when the surface layer has been removed. This, however, should be done somewhat sparingly, thus giving the greater prominence to the exhibition of those appearances most commonly displayed by the shells as we find them on the beach or dredge them from the sea. Very small and delicate shells may be mounted on cards, as suggested for other objects; but, as a rule, the specimens are best displayed by simply placing them on a layer of cotton wool in shallow boxes of convenient size.