Geological history.
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.
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.
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.