The Unios are most abundant in clear, running water, where the bottom is gravelly or sandy. The interiors of the shells are iridescent, and vary greatly in tint, exhibiting many delicate shades of color from silvery white to straw color, pink, purple, brown, etc.
About five hundred species of American fresh-water mussels have been recognized by conchologists. Many of these differ from one another so very slightly that they are scarcely distinguishable from an examination of the shells themselves, or even from the descriptions, and a detailed index to the complete list is of little economic importance. The professional fishermen and the shell-buyers take the trouble to name only the species with which they deal, which includes only about twenty-five species, all of which are margaritiferous, though some to a greater extent than others. In the pearling regions a popular nomenclature exists, the names given by the fishermen having reference to the shape, color, etc.
The niggerhead (Quadrula ebena) is the most numerous in the Mississippi, and it is extensively used in button manufacture. The thick shell of this species is almost round, with a black outer surface and a pearly white interior. At maturity it averages about four inches in diameter and four ounces in weight. Owing to its uniform whiteness and the flatness of its surface, it is well adapted to button manufacture, and for this purpose more than twenty thousand tons are taken in the Mississippi Valley every year. When the fishery originated, the niggerhead was very abundant in some places, and especially between La Crosse and Burlington. From a single bed near New Boston, Illinois, measuring about 200 acres in area, 7500 tons, or about 70,000,000 individual shells, were removed in three years. In 1897, a bed of 320 acres near Muscatine furnished 500 tons, or about 4,750,000 shells. This species occasionally yields valuable pearls.
Two species of Unios, Quadrula undulata and Q. plicata, are known among the fishermen as “three-ridges.” The former is also known as the “blue-point” from the fact that the sharp edge is usually tinged faint blue on the inside. Although not the best for button manufacture, the shells yield the greatest number of pearls.
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.