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Marine shellfish live in all kinds of places below high water mark; and some of the semi-amphibious ones thrive even above ordinary high water mark, where for days at a time nothing but the tops of the waves could reach them. They are found on seaweed and on rocks, and on sand or mud-banks; but especially in places near rocks on marine grass banks bare at low spring tides. Some live on the surface of the water, some burrow in sand or mud, and some bore holes for themselves in the softer rocks. Some live in deep water; but the better coloured shells are found near low water mark, or in shallow water; for light is as necessary to the perfecting of colour in shells as in flowers. Shells that have grown in a harbour are more fragile than those grown in the ocean, and are usually less brilliant in colour, as harbour water is not as clean as ocean water. The colour of shells (as of insects) depends largely on environment, and is only one, and by no means the most reliable, method of deciding the species. An expert can at a glance tell whether a given shell has come from shallow or deep water, and whether from an exposed or sheltered spot. Most shellfish move about a great deal, and migrate into deeper water in summer; and on bright clear days retire into dark corners amongst, and even under, stones. On a dull day a collector is frequently more successful than on a bright, sunny day; and in spring or early summer the best hauls of live shells can be made. Nearly all shells have an epidermis, or outer skin. In some this is very apparent, as in the Lotorium olearium (Plate V., Fig. 1), or the Solenomya parkinsoni (Plate IX, Fig. 18), while in others it is nearly transparent, and hardly perceptible. To enable the true colours of a shell to be seen the epidermis must be removed.
The supposed original form of a shell was that of a volute univalve, such as the Triton (now Lotorium), or Struthiolaria. To properly enclose the animal, and make it safe from enemies, an operculum, or lid, was so formed that when the animal retired into the shell this filled up the opening. The operculum is usually like a piece of thin, rough brown horn, and where no reference is made to an operculum in this work, it must be understood that the operculum is horny. Some shells, such as the Astralium sulcatum (Plate VI., Fig. 18), and the Turbo helicinus (Plate VI., Fig. 17), have a shelly operculum; that of the latter being the well-known cat’s eye.
In some shells the operculum is small, in others large, and progressing step by step we find some, such as the scallop and oyster,
