Page:Natural History, Mollusca.djvu/292

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tants of sandy shores in most seas, and the majority of them are found in the vicinity of low-water mark. Many of them are of a large size. We have a British species, Mactra helvacea, which attains the size of three inches by two and a half in diameter. This, however, is one of the rarest of our native shells, as M. stultorum is one of the most abundant. The latter is found on every sandy beach; it is about half the size just mentioned, somewhat triangular in outline, and varying much in colour, from plain drab to fawn-colour, marked with pale zones and white divergent rays.

Family Donacidæ.

The shell is more or less triangular and compressed, broader than long, equal-valved, but unequal-sided, the hinder side being the shorter. The texture is strong and compact, the hinge variable, but always provided with conspicuous primary teeth.

The animal is generally richly coloured, with the mantle usually fringed; the siphonal tubes are separate and much developed, having their orifices adorned with deeply-cut tentacles. The foot is large, compressed, sharp-edged, and angular; the gills very unequal.

The species of this family, which are rather numerous, occur in all parts of the world, but most abundantly in the Southern Hemisphere. They live buried in sand or sandy mud, with the short side of the shell uppermost, the siphons being protruded, and range from low-water mark to a depth of ten fathoms.