is the great receiving port for these, and many tons are used annually. In diving for pearls the Ceylonese, who are able to remain beneath the water several minutes, place as many shells as possible in a basket and then ascend, leaving the crew to haul the basket up. In Lower California many divers of to-day go down in armor. [Illustration: FIG. 85.--Pearl oysters.] Pearls are generally valued according to their symmetry and color. Some are perfect, and when of large size bring vast sums. One of the shahs of Persia owned a necklace in which the pearls were perfect and as large as hazelnuts. The pearl is the result of the oyster's attempts to protect itself from injury. If we should take one of these beautiful pearl oysters and with a gimlet bore a hole through the shell from the outside and replace it in the water, we should find, months after, if the oyster was examined, that it had, by using its mantle, secreted a large amount of pearly nacre over the wound, not only filling up the hole, but heaping the pearly secretion over it until a projection a quarter of an inch high was the result, resembling a pearl attached to the shell. This is the way imperfect pearls are formed; they are the attempts on the part of the oyster to prevent injury to itself. Occasionally some foreign body, like a grain of sand, will enter the shell. Its sharp edges will cut the soft flesh of the delicate creature, which immediately covers it with pearly nacre. The larger it grows the more the oyster notices it among its folds, and the more it instinctively covers it with pearl. In this way the pearls grow. The seed pearls are those in which some impurity has been covered but a few times, while the very large pearls are those which have been bathed in nacre time and again. If a large pearl is cut in halves, the various layers can be counted, the sections recalling the interior of an onion. The skillful native fakirs of the East take advantage of this industry of the pearl oyster to introduce metal beads and figures of the Buddha into shells, which are then marked. The objects finally become covered, when they are removed from the shells and sold to the unsuspecting natives as "miracles." One of the interesting shells of the seashore is the Pinna. I have found the shores of the outside islands of Texas scattered with them. They are also called fan shells, and are attached to the bottom by a peculiar cable, or byssus, formed of a silklike substance which has been woven. Gloves and hose of pinna silk may be seen in the British Museum. [Illustration: FIG. 86.--Pectens swimming.]