We have now come to the twentieth family, the Aviculidæ, which contains Avicula, Malleus, Maleagrina, Perna, and Pinna. The shells of the Hammerheads (_Malleus_) have a rough resemblance to the implement from which they derive their name. The valves are nearly equal, blackish, and somewhat wrinkled on the exterior, often brilliantly nacred in the interior. They are enlarged to the right and left of the hinge, forming prolongations on each side, which give them the fancied resemblance to the Hammerhead (Fig. 163). At the same time they grow in a direction opposite to the hinge, which gives something approaching the handle of the implement. This is the first feature which a glance at _Malleus alba_ (Fig. 162) conveys. The hinge is without teeth, having instead a deep conical fossette or dimple, for the reception of a very strong ligament which acts upon the valves. The animal is contained in the interior of the shell, its mantle fringed by very small tentacular appendages. Only six actually living species of the genera are known, which are inhabitants of the Indian Ocean, of the Australian seas, and the Pacific Ocean. The beautiful diaphanous nacre which embellishes the interior of so many ornamental cabinets are principally produced by the animal inhabiting the _Meleagrina_, a bivalve, sometimes designated the _pintadine_, or mother-of-pearl shell. This bivalve moors itself to the bottom of the sea by a strong byssus of a brownish colour. The door-posts of the shells are irregularly rounded in their young days; they are externally lightly foliated, and ornamented with bands of green and white, which spring from the summit in rays, and afterwards break off into two or three slightly scattered branches. In old age they become rugged and blackish. The shell is in its perfection when about eight or ten years old, their size being then about six inches in diameter, with a thickness of about an inch and a quarter. Nacre is the hard and brilliant substance with which the valves of certain shells are lined in the interior. This substance is white, silky, slightly azure, and more or less iridescent. Most of the bivalves are supplied with nacre; some of them even yield a blue, or blue and violet pigment. The iridescent _Haliotis iris_, for instance, is an emerald-greenish blue of changing colour, with reflections of a purple violet. _Turbo argyrostomus_ (Linnæus) presents a mouth of bright silvery hue, while _Turbo chrysostomus_ appears in all the glory of gold; but the Pintadine yields the purest white nacre, as well as the most uniform, and especially the thickest. This product owes its brilliant and delicate appearance to the play of light on it in its highly-polished state. For practical purposes the nacre is separated from the shell with an instrument; sometimes all the exterior part of the shell being dissolved away from the precious substance, leaving only the naked bed of nacre. [Illustration: Fig. 164. Meleagrina margaritifera (Linnæus).