a rough, warty exterior. The supply is small and uncertain.
Another
rare species is the butterfly (Plagiola securis). It is a small, flat,
thick shell of fine color, and the valves are butterfly in shape with a
reddish-brown epidermis striped by darker radiating lines. It is
abundant only in the Illinois and Ohio rivers.
The
hatchet-back, hackle-back, or heel-splitter (Symphynota complanata), is
a large black mussel having a thin sharp-edged shell, one valve-edge
projecting. It yields few pearls though fine specimens are occasionally
found in this variety.
The
blue-point (Quadrula undulata) has a large, thick shell, with ridges on
the exterior, curving round the umbones and extending to the edge. Like
the black-edge meleagrina, the nacre at the edge is discolored. In this
case by a bluish or purplish tint.
Some
idea of the enormous quantities of mussels contained in some of these
beds in our western rivers may be gained from the reports of the
fisheries in the first years of their disÂcovery. Ten thousand tons of
shells were
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