ring
in the rivers of America, Scotland, Saxony, Bavaria, Norway, Sweden,
Russia, France, China, etc. These mollusks exist exclusively in the
fresh-water streams, lakes, and ponds, and quickly die when submerged
in salt water. The Unionidœ are of particular interest in
America, as it is here that this group is most abundant, and nearly
every stream east of the Rocky Mountains contains more or less of them.
The Mississippi basin abounds in Unios, or "clams," as they are known
to the fishermen of that region, and furnishes about 400 of the 1000
recognized species of this important family.
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, Quadrilla 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.
