The Freshwater Mussel (Unio) has long been known as one of those shell-fish that produce pearls; but those procured from the present species are commonly small, ill-coloured, and of little value, though they have been at various times much sought for. The passage in Camden's "Britannia," about the pearls in Cumberland, evidently refers to these. "Higher up the little river that runs into the sea, . . . the shell-fish, having by a kind of irregular motion taken in the dew, which they are extremely fond of, are impregnated and produce pearls, or to use the poet's phrase, baccæ cochleæ, shell-berries, which the inhabitants, when the tide is out, search for, and our jewellers buy of the poor for a trifle, and sell again at a very great price." The attempt to account for the origin of pearls by the drinking in of dew-drops, my readers may probably think more poetical than philosophical. A very curious account of a recent pearl-fishery in North Wales is given by a correspondent in "Loudon's Magazine of Natural History" for 1830. The writer has confounded the Mytilus edulis with the Unio. To the latter only his remark on pearls "found up the river" applies. His account is as follows, with some slight omissions:—
"Mussels are kept in many places in artificial beds, to be used when required for bait. At Anstruther, in Fifeshire, we have seen these 'mussel gardens,' as they are called—little plots of seashore between tide-marks, edged in by stones, and held as private property. In Northumberland, Mr. Alder states, the fishermen build up piles of stones among the rocks, to keep their mussels safe."
- Brit. Mollusca, ii. 174, et seq.