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 hidden in invisible vaults, but were sedulously spread abroad in the most conspicuous places, and by the sides of the public ways." Hence we may add, the "_Siste Viator_" (traveller, stop!) so common upon tombs to this day. But why are not tombs placed by the roadside in our times? "It would seem," says the writer just quoted, "as if these mementos of mortality were not so painful or so saddening to Pagans as to Christians; and, that death, when believed to be final dissolution, was not so awful or revolting as when known to be the passage to immortality. I pretend not to explain the paradox, I only state it; and, certain it is, that every image connected with human dissolution, seems now more fearful to the imagination, and is far more sedulously shunned, than it ever was in times when the light of Christianity had not dawned upon the world."[13] [13] Rome in the Nineteenth Century, vol. ii. letter 36.