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PINNA OR WING SHELL (Pinna seminuda) One third natural size

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.