one
distinctive feature which places them in the class under consideration
is the possession of a nacreous lining to the shell, for no shell fish
can produce a true pearl without it. The fresh water pearl-bearing
mollusk is a mussel, unio margaritifera, also found in many varieties,
but all characterized alike by the nacreous lining of the shell.
These
creatures, living upon the earth where water always covers it, create
in the building of their habitations a material of great beauty, and
sometimes produce gems which princes covet. Of the most delicate
nature, they build for themselves out of the water by which they are
surrounded, houses strong and enduring, fitted for their protection
from the rough chances of life, yet so furnished within that they
suffer no inconvenience from the rugged strength which encloses them.
Few things are coarser than the exterior of these domiciles, but
nothing in nature is finer or more exquisitely beautiful than the
substance with which they are lined.
The
avicula margaritifera is a habitant of the coral reefs and shoals about
the islands and shores of the tropics; there are none living
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