Several species of Horse Mussels inhabit our shores, from low-water mark to a depth of fifty fathoms, but none of them is used for food. The commonest species is Modiola modiolus, which has a particularly strong byssus, and its fibres generally bind together such a number of stones &c. that the shell is completely hidden in the entangled mass. Other British species include M. barbata, so called on account of the peculiar fringed threads of the epidermis; M. phaseolina, in which the epidermis threads are not fringed; and M. tulipa, named from the streaks of crimson or purple that radiate from the umbones of the shell and remind us of the colouring of the tulip flower.
An allied sub-genus (Crenella) includes a few small British molluscs the shells of which are crenulated on the dorsal margin behind the ligament. The shells are short and swollen, and lined by a brilliant pearly layer. One species (C. discors) is pale green, with radiating lines from umbo to margin. It is common on many of our shores, but is not easily found, as it hides at or below low water mark, in a nest formed by binding together small stones. Other species, one of which is black, are less abundant, and are not readily obtained except by the use of the dredge.
Before leaving this family we must refer to the remarkable Dreissena polymorpha, sometimes called the Chambered Mussel, on account of the chamber which is formed in the beak of the shell by means of a pearly plate that stretches across it. This animal is not indigenous to Britain, but was introduced from the East by trading vessels, either attached by its silken byssus to timber that had been left floating in water previous to being shipped, or to the bottoms of the ships. It seems to thrive almost equally well in salt, brackish, and fresh waters, and has spread very rapidly since its introduction. It is more commonly found, however, in docks, canals, and rivers, and is on that account usually described with the fresh-water species.
Fig. 151.—Dreissena polymorpha
The form of the shell is very similar to that of Mytilus, but has no internal pearly layer, and the valves are bluntly keeled. The mantle is closed, the siphons short, and the foot small.
Our next family—the Aviculidæ—contains those shells that are distinguished by peculiar flat processes on each side of the umbones, one of which, the posterior, is generally wing-like in form. They are popularly known as Wing Shells, and the family includes the so-called Pearl Oysters. Most of the species are natives of tropical seas, but several are common on our own shores.
