Mollusca made their appearance on the globe at a very early epoch in the history of the development of animal life, a large number of fossil forms, such as _Nautilus_, _Lituites_, _Orthoceras_, &c., being found in the oldest Palæozoic formations. Probably all these belonged to the _Tetrabranchia_, of which one descendant only, the Pearly Nautilus, has survived to our period. Some Gastropods and Bivalves coexisted with those ancient Tetrabranchs; but these types abounded more in the later geological epochs, many Tertiary forms being undistinguishable from species which now exist. [Sidenote: General distribution.] The greater number of Mollusca are inhabitants of the sea, some passing their whole life at the surface hundreds or thousands of miles away from land; others at the bottom of the ocean at all depths, some having been dredged at five miles from the surface. Many are found in much shallower water, and a large number between tide-marks. Rivers and lakes furnish an immense variety of forms, and vast numbers live on land in all situations—on mountains, in valleys, forests, and deserts. Molluscs are either animal or vegetable-feeders, the former preying principally upon other members of their own class. [Sidenote: Systemic arrangement.] The following Table shows the systematic arrangement of the Mollusca adopted in the Shell Gallery:— Class I.—Amphineura. Order 1.— Polyplacophora. Chitons. „ 2.— Aplacophora. Neomenia, Chætoderma, etc. Class II.—Gastropoda. (Section Streptoneura.) Order 1.— Scutibranchia. _Nerites_, Top-shells, Ear-shells, Limpets. „ 2.— Pectinibranchia. Rock-snails, Whelks, Olive-shells, Harp-shells, Cones, Strombs or Wing-shells, Periwinkles, Carrier-shells, etc. (Section Euthyneura.) Order 1.— Opisthobranchia. Bubble-shells, Sea-hares, Umbrella-shells. „ 2.— Pulmonata. Land and freshwater Snails, False Limpets. Class III.—Scaphopoda. Tooth-shells (_Dentalium_). Class IV.—Lamellibranchia. Bivalved Molluscs. Order 1.— Protobranchia. Nucula, etc. „ 2.— Filibranchia. Anomia, Common Mussel, Ark-shells, etc. „ 3.— Pseudolamellibranchia. Pearl-Oyster, Hammer Oyster, Wing-shells, etc. „ 4.— Eulamellibranchia. Freshwater Mussels, Cockles, Razor-shells, Ship-worms, etc. „ 5.— Septibranchia. Poromya, etc. Class V.—Cephalopoda. Order 1.— Tetrabranchia. Pearly Nautilus. „ 2.— Dibranchia. Octopus, or Poulp, Argonaut, Squids, and Cuttle fishes. Class I.—AMPHINEURA. [Sidenote: Cases 1–3.]