A species somewhat similar to the niggerhead is the bullhead (Pleu-robema œsopus). 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 oer 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 com-planata). On the Atlantic seaboard, the principal species in which pearls have been found are Unto complanata; the Alasmodon arcuata, which has hinge teeth, and a species of Anodon. Pearls from the Unto complanata are usually smaller but more lustrous than those from either of the other species.
Among
the many fresh-water mussels are found some remarkable conditions of
animal life. Probably the most curious is the parasitic stage of
certain species. When hatched from the egg, each one of these
is provided with hooks or spines, by means of which it attaches itself
to the gills or fins of a swimming fish and becomes embedded
