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44 0STREID.E. ,
separated by a wide chasm. The shell is remarkably
calcareous, and consists for the most part of layers
termed by Dr. Carpenter " sub-nacreous " and having
comparatively little adhesion one to another. These layers
are internal. The outer layers are composed of pris-
matic cellular structure, and have no natural cohesion.
The weight of the animal in a full-grown example is
to that of the shell. The late Mr.
very disproportionate
Thompson of Belfast ascertained that a large oyster
from that bay weighed altogether two pounds, but that
the weight of the animal taken out of the shell was only
an ounce and a half. Large-sized specimens from the
British seas seldom exceed six inches in length ;
but on
the North-American coast this species (if it be the same
as ours) is said to attain occasionally twice that size.
Young shells are sometimes marked with radiating
and now and then one is found attached
purple streaks ;
to the operculum of a living Buccinum undatum,
the sur-
face of which it completely covers and takes its form.
Before adverting to the economical point of view, I
may mention some of the minor uses to which oysters
are put. These are few : they serve to keep an aqua-
rium free from the spores of sea-weeds ;
their shells are
burnt as a substitute for lime ; and formerly certain
medicines were prepared from their calcined material.
Also pearls of inferior lustre, often small and of an
irregular shape, are obtained from them. Antiquaries
tell us that the shells have been discovered in Saxon
tombs, and that in still older places of sepulture in the
Orkneys they
are found drilled in such a manner as to
show that they probably formed articles of personal
ornament. They must have made a clumsy necklace.
But their chief value results from the fisheries, which
for more than eighteen centuries have rendered Great