One third natural size ] [Illustration: PEARL-OYSTER OF CEYLON (_Margaritifera vulgaris_) Natural size ] A species somewhat similar to the niggerhead is the bullhead (_Pleurobema æsopus_). This shell is thick and opaque, the nacre is not so iridescent as that of the niggerhead, nor does it yield pearls of such good quality. These two species are not evenly distributed over the bottom of the streams, but occur in great patches or beds, sometimes several feet in thickness and covering many hundreds of acres. Some of the beds are several miles in length, and they may be separated by twenty or thirty miles in which the mollusks are so scarce that profitable fishing can not be made; but usually the reefs are smaller and more closely situated. The sand shells (_Lampsilis_)—of which there are several species—do not occur in large beds, but are scattered over the sandy beaches and sloping mud-banks. In shape they are narrow and long, adults measuring five or six inches in length. Owing to the small waste in cutting, due to uniformity in thickness, these shells are sold to button manufacturers for more than the niggerhead, which in turn is more valuable than the bullhead. The buckhorn (_Tritigonia verrucosa_) is very long and narrow; on the dark brown exterior it is rough, as is the horn from which it takes its name, while the interior shows a beautiful display of colors. This is not found in beds, but lies scattered among other species. It sells at a relatively high price—usually in excess of $20 per ton—for button manufacture. Another species is the butterfly (_Plagiola securis_), which is very prettily marked on the outside with faintly colored dotted stripes of varying length. Over a background of dark yellow run black stripes to the outer edge of the shell, with dark dots between the stripes. The shell is small and thick, and like the sand shell and the buckhorn, is found in small quantities. Owing to the beauty and permanency of its luster, this shell is in demand for button manufacture, and its pearls are often very beautiful. Other well-known species are the pancake (_Lampsilis alatus_), the maple-leaf (_Quadrula wardi_), and hackle-back (_Symphynota complanata_). On the Atlantic seaboard, the principal species in which pearls have been found are _Unio complanata_; the _Alasmodon arcuata_, which has hinge teeth, and a species of Anodon. Pearls from the _Unio complanata_ are usually smaller but more lustrous than those from either of the other species.