In wall-cases on the west side of the gallery is a series of shells arranged on tablets to show the leading structural types, such as univalve, bivalve, multivalve, etc.; then the nature of the outer coat, or “skin”; and, thirdly, some of the more striking styles of ornamentation and colouring. Following these, a division is devoted to the display of the general form of the shells of bivalves, special attention being directed to the nature of the hinge by which the two valves are joined. Fresh-water Mussels (_Unionidæ_) are selected as examples of great variability in the form of the shell in closely allied species. Near by is a small series of the shells of boring bivalves, many in the various substances they perforate. Alongside are shown in a similar manner modifications in form and structure presented by Univalve or Gastropod shells; many of the shells having been cut to show their internal structure. Specimens of the horny or shelly plate (operculum) closing the mouth of many Gastropod shells are also exhibited. In one compartment are displayed spirit-preparations of Cephalopod Molluscs, such as Octopus, Cuttle-fish, Squids, Nautilus, Argonaut or Paper-Nautilus, etc. Specimens of the horny beaks possessed by all members of this class are also exhibited; and a Pearly Nautilus (fig. 31), with the shell cut in two in order to show the air-chambers and the comparatively small space occupied by the “animal,” will be found of special interest. From the roof are suspended life-sized models of a Giant Squid (_Architenthis_) and a Giant Octopus. The Cephalopods were extraordinarily numerous in past ages, and many of the fossil forms are exhibited in the Geological Department. [Illustration: FIG. 29.--THE WHELK (_Buocinum undatum_), a Gastropod Mollusc. _a_, siphon; _b_, foot; _c_, tentacles; _d_, eyes; _e_, operculum.] The main shell-collection is contained in four parallel rows of table-cases, the arrangement commencing on the right as the gallery is entered. The first two rows contain the marine forms of Gastropods--a division which includes Snails, Slugs, Whelks (fig. 29), and all those Molluscs which crawl upon the under surface of their bodies; the Cowries, Cones, Volutes, Mitras, and Murexes forming some of the most attractive groups. The two rows of cases on the left contain Land-Shells, Bivalves, and Cephalopods. The Cockles, Oysters, Clams, Piddocks, Teredos, Scallops, and Ark-Shells represent some of the principal types of Bivalves, so called on account of their shells being formed of two pieces or valves. A Giant Clam (fig. 30), at the S. end of the gallery, weighs 310 lbs., and measures 36 inches in length. [Illustration: FIG. 30.--LEFT VALVE OF THE GIANT CLAM (_Tridacna gigantea_), a Bivalve Mollusc.] [Illustration: FIG. 31.--THE PEARLY NAUTILUS (_Nautilus pompilius_). _a_, body; _b_, funnel; _c_, eye; _d_, hood; _e_, tentacles; _f_, muscle of attachment to the shell; _g_, siphuncle.]