The linings of some have a black rim, extending from the hinge on one side, around the edge to the hinge on the other side. Viewed from the edge this dark band appears to be a sixteenth to half an inch wide (widest at the lip), fading out as it becomes lost under the thicker white nacre of the interior, but turn the shell up and look at it squarely from the front and it is black only around the extreme edge where it joins the epidermis. This kind of shell is found in the Pacific about the islands of Polynesia and is called the black shell. In others the nacre is white to the edge. The iridescence of the white shell generally shows more play of color than that of the black. The white shell is usually somewhat flatter and broader than the black, and the epidermis is light yellowish-brown. This variety is found in great abundance on the northern and western coasts of Australia. The yellow, greenish and grayish shells (these colors refer to the edge of the lining), are similar in every way, but inferior, the yellow being the best of the three. The shell lining of a common form of the unio, or fresh-water mussel pictured at page 146, like that of the meleagrina, shows little iridescence except at the edges outside the pallial lines, where the nacre is comparatively thin, and at the striated surface of the scar or bed of the adductor muscle. In quality of color and luster it is inferior to the nacre of the sea fish, the white being more chalky in appearance and the luster less pearly. The material of which the shell is composed and its construction are however almost identical with that of the salt-water mollusk. In fact all shells are made of the same ingredients and are constructed on the same general principles by the animals inhabiting them. [Illustration: MISSISSIPPI NIGGER-HEAD PEARL MUSSEL] This description of pearl shells has been given here because a knowledge of the shell enables one to understand the formation and characteristics of a true pearl, and the differences which exist between the gem and other similar formations formed in pearl and other oysters, mussels, and univalves. Many such formations are found, having the elements and constructed like one or both of the outer parts of the shell, and some, in part like the lining, but these are not true pearls; the gem has neither the material nor construction of the middle and outer shell. Except that the pearl, because of its form, is rarely iridescent even to a slight degree, whereas the nacreous lining of some pearl-bearing shells is brilliantly so, the pearl and the nacre of the shell in which it grows, are essentially the same. Pearls are more or less spherical and independent formations, made by the fish on the same plan and from the same secretions with which it lines the shell, misdirected by abnormal conditions. Those constructed like any other part of the shell are not true pearls.