fact that suggested to Linnaeus his
method of producing pearls at pleasure, by puncturing the shell with a
pointed wire. But this explanation accounts only for the origin of
such pearls as are attached to the shell; while the best and greatest
number, and, indeed, the only ones which can be strung, have no such
attachment, and are formed in the body of the animal itself. 'The
small and middling pearls,' says Sir Alexander Johnston, 'are formed
in the thickest part of the flesh of the oyster, near the union of the
two shells; the large pearls almost loose in that part called the
beard.' Now, these may be the effect merely of an excess in the supply
of calcareous matter, of which the oyster wishes to get rid; or, they
may be formed by an effusion of pearl, to cover some irritating and
extraneous body." The reality of the latter theory is strengthened, if
not proved by the Chinese forcing the swan muscle to make pearls by
throwing into its shell, when open, five or six minute mother-of-pearl
beads, which, being left for a year, are found covered with a crust
perfectly resembling the real pearl. Such is one method of getting
artificial pearls. The extraneous body which naturally serves for the
nucleus, appears to be very often, or, as Sir E. Home says, always, a
blighted ovum or egg. This theory which, however, is here but partly
explained, has been fully adopted by Sir E. Home:--"if," says the
enthusiastic baronet, "I shall prove that this, the richest jewel in a
monarch's crown, which cannot be imitated by any art of man, either in
the beauty of its form or the brilliancy and lustre produced by a
central illuminated cell, is the abortive egg of an oyster enveloped
in its own nacre, of which it receives annually a layer of increase
during the life of the animal, who will not be struck with wonder and
astonishment?" And, we must add, that the proofs are very much in
favour of this conclusion.

[12] The writer of An Introduction to the Natural History of
Molluscous Animals, in a Series of Letters: one of the
most delightful contributions to the _Magazine of Natural
History_, since the establishment of that valuable
journal.

* * * * *

ROMAN TOMBS.

"Tombs," observes the clever author of _Rome in the Nineteenth
Century_, "formed a far more prominent feature in ancient communities
than in ours. They were not crowded into obscure churchyards, or