The oysters are seldom found in water with a temperature below 75 degrees and they seem to thrive best in warm sheltered bays and inlets, especially when the banks are situated far from the equator. They attach themselves to the beds by a bunch of tough threads which pass out through an aperture in the shells, near the hinge, and fasten on the rocks and stones; consequently the oysters do not lie flat, as might be supposed, but maintain an upright position, hinge down, lip end up, and the shell slightly open for the passage of the food-laden water, as the fresh-water mussels do. These threads are called the beard or byssus, and are composed of material similar to the epidermis of the shell. The abalone, which is a univalve, holds on to the rocks by the foot, a flat muscular appendage used for locomotion and also as an anchor on the principle of the leather toy known to boys as a sucker. Although pearls of value are found only in shells containing mother-of-pearl, a small proportion only of the mother-of-pearl shells contains pearls, and many varieties in which pearls are found do not yield enough nacre to make the shells valuable. The size of the meleagrina in some seas is remarkable. That at page 127, photographed from a Tuamotu shell, measures 8-7/8 inches by 6-7/8 inches and weighs twenty-eight ounces troy. It is of the black-edge variety, contains a large quantity of fine quality mother-of-pearl, and has a beautiful small pearl attached to the lining near the center of the shell. Though large, it is not full grown. It is probably twelve to fourteen years old and would continue to lay on mother-of-pearl and so grow thicker and heavier until sixteen to eighteen years of age, when the oyster would reach maturity. The Australian white shell at page 129 is a young shell—that is, it has not attained the full thickness and weight of a mature shell. The shells at pages 131 and 161 are from the coast of Venezuela; they measure 2-1/4 by 2-1/4 inches and weigh seven pennyweights each. The common form of the pearl-bearing fresh-water mussel unio (nigger-head) is illustrated at page 146. This shell measures 3-3/4 by 2-3/4 inches and weighs 3-1/2 ounces. It is from the Middle West of the United States. In construction it resembles the meleagrina, the epidermis being dark, though not as rough as that of the oyster, and the lining white, showing slight iridescence around the lip-edge and to a greater degree on the adductor muscle scar. The mother-of-pearl under the epidermis at the thick or hinge end is quite iridescent, and the lines which make the color play are plainly discernible under the loup.