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. [Illustration: 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. [Illustration: FIG. 152.--_Avicula_, AND _Pinna pectinata_] One species of the typical genus is sometimes found off the coasts of Cornwall and Devon. The shell is very oblique, and the valves are unequal, the right one, on which the animal rests, being somewhat smaller than the left; and the epidermis is very scanty. The hinge is long and straight, without teeth, and the cartilage is contained in grooves. The interior of the shell is pearly. The posterior adductor impression is large, and not far from the middle of the shell, while the anterior, which is small, is close to the umbones. The mantle of the animal is open, and the margins of the lobes fringed; and the small foot spins a powerful byssus.