The gills in this order are plicate, and the two lamellæ of each plate are furnished with conjunctive or vascular interlamellar junctions, and the filaments are connected by interfilamentar ciliated discs in some cases, in others by vascular concrescence. The mantle-margins are separated all round, and the foot is either small or wanting. Only a single adductor muscle is generally present. The Pearl-oysters, true Oysters, and Scallops are the forms which constitute this order. The large family of _Aviculidæ_ includes the “Wing-shells” (_Avicula_), the “Pearl” and “Hammer Oysters” (_Meleagrina_ and _Malleus_), and _Pinna_ (Fig. 28). Some species of _Pinna_ attain to a length of two feet. They are found imbedded in the sand with the narrow pointed end downwards. They form a large silky byssus, which can be woven or knitted into gloves, socks, etc. (see side table—case B). The “Hammer-Oyster” (_Malleus_) is so called from its rude resemblance to a hammer. The “Pearl-Oysters” (_Meleagrina margaritifera_, Fig. 29) possess rather heavy strong shells, lined with very thick layers of “mother-o’-pearl.” Hundreds of tons of these shells are annually collected at the great pearl-fisheries of North and West Australia, and imported into Europe. The pearl-oyster of Ceylon (_M. fucata_, Case 146) is a smaller species, and collected more for the pearls than the shells. The round pearls, which are valued so highly, are either excrescences of the pearly layer or are found loose in the fleshy parts of the animal. Some small foreign body which has accidentally penetrated under the mantle and irritates the animal is covered with successive concentric layers of nacre, thus attaining sometimes, but rarely, the size of a small filbert. The nacre is generally of the well-known pearly-white colour, very rarely dark, and occasionally almost black. The action of the animal in secreting successive layers of nacre over any foreign body which intrudes between the mantle-folds, and thus converting it into a pearl, is strikingly illustrated by two specimens in which, in the one case, an entire fish, and in the other a small crab, have been so enclosed (see side table-case E). [Illustration: Fig. 28. British “Fan-Mussel” (_Pinna pectinata_): _a_, the byssus. Case 150. ] [Illustration: Fig. 29. Pearl-Oyster (_Meleagrina margaritifera_). Case 146. ]