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Family Aviculidae

SHELL oblique, valves unequal, right one smaller, always underneath; hinge line straight, much elongated; umbones eared, one ear wing-like; animal attached by byssus; foot small; mantle lobes fringed, free. Family mostly extinct; one thousand fossil species known, chiefly in Paleozoic strata. Of One hundred and twenty living species, not one is found in northern latitudes where fossils abound. Confined now to warm, temperate and tropical seas.

Genus AVICULA, Lam.

Shell oval, eared, posterior ear produced into a wing-like extension. Body small; byssus large. Many species secrete pearls. Twenty-five living and three hundred fossil species. The typical forms resemble in outline the profile of a bird in flight.

The Saffron Wing Shell (A. crocea, Lam.) has a long, slender wing-like extension of the hinge, almost twice the length of the oblique valves. The short anterior wing completes the bird figure, supplying the head and beak. This was Lamarck's type species. Who wonders that he called the genus Avicula-"little bird"? The trim outline of this elegant shell is that of a swallow, built for swift flight. Length, 4 to 5 inches.

Habitat.-Philippines.

A. heteroptera, Lam., has roundish, unequal valves, and long, tapering, dissimilar wings. Colour, bright chestnut, obscurely rayed. Length, 3 to 4 inches.

Habitat.-New Holland.

The Great-winged Avicula (A. macroptera, Lam.) combines unusual size with elegance of form. The straight, narrow wings extend like keen stillettos in opposite directions from the central hinge. The dusky valves are thin and marked with pale radiating lines. Young shells are adorned with minute, fimbriated scales all over the surface. These disappear later. The pearly lining is beautifully iridescent. Length, 5 to 8 inches.

Habitat.-West Indies, Moluccas.

The Atlantic Wing Shell (A. Atlantica, Lam.) is reddish brown with numerous undulating wrinkles, separated into radiating series by white lines spreading from the umbones to the margins. The wrinkles are set with sharp spines, which are soon worn off with the fibrous epidermis. Wing, a straight, narrow blade. European forms, thin, pale, flat, long-winged and spiny, are a contrast to our thick, dark-coloured, smooth ones, with short wing and rounded, concave valves. Length, 3 to 4 inches.

Habitat.-Mediterranean, northward. North Carolina to Florida and West Indies.

The Bearded Wing Shell (A. barbata, Rve.) is orbicular in outline, with a straight hinge line, and no wing extension. This yellow shell bears series of concentric laminae, which change near the margin into a series of regularly overlapping, spoon-shaped lobes, forming a heavy fringe. Diameter, 3 to 5 inches.

Habitat.-Panama.

The Emerald Wing Shell (A. smaragdina, Rve.) an elongated oval form, almost without a hint of a wing behind, is smooth and a rich dark green in colour. Yet it swings off very obliquely from the hinge, and is recognisable as an Avicula in spite of its unusual form.

The Great Pearl Oyster (A. margaritifera, Lam.) better known to the commercial world by its sub-generic surname, Meleagrina, is a wing shell with wings reduced to small angular projections near the hinge of the large, flat circular valves. The outside surface shows a coarse, laminated structure, of dull olive to smoke colour. Within, the thick pearly lining is a beautiful, iridescent expanse, interrupted by a central muscle scar.

When small, the valves are ribbed and wear a luxuriant growth of long, flat scales, twisted and curved like fronds of coral. These disappear as the shell grows larger and thicker. At maturity it is often ten to eighteen inches across and surprisingly heavy.

Pearl oysters live in tropical seas on clean, sandy bottoms fifteen to twenty fathoms down. The strong byssal cord thrown out through the hinge anchors the mollusk to coral masses or to other shells, whence they never move until brought up by divers. These are the bivalves which for centuries have furnished the precious "oriental pearls," adding constantly to the treasure of kingdoms and individual collectors.

Mother-of-pearl, the thick lining of each valve, is the dependable product of pearl fisheries. It is secreted in annual layers by glands in the mantle. There is just enough animal substance in it to support the particles of lime carbonate. The iridescence is due to microscopic undulations of the various layers which compose it. These layers expose oblique edges to the surface, and the refraction of light produces the rainbow colours.

The best mother-of-pearl comes from healthy, full-grown shells; the finest pearls from shells distorted by crowding and disease, and invaded by parasites and foreign particles. The first is a natural growth; the second abnormal. A lusty, wellfed mollusk, enjoying life, has a neighbour, warped, debilitated, suffering, with a grain of sand rolling around in its mantle folds. Coat after coat of nacre is added to this irritating foreign body to lessen its injury to the tender flesh. When the diver finds it, a magnificent pearl, it makes him rich. Only one shell in a thousand, we are told, contains a gem of any value. So the lines of Browning would do little, I fancy, to reconcile a discontented pearl-diver to his hard lot: