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called Alasmodonta margaritifera, is markedly distinct from the other species ; in its superior size, often five and a half inches long and two and a half wide, and one thick,-it is thus proportionately much longer and more compressed ; in its strong and pitch-black epidermis, and in the adult specimens by the posterior tooth being obsolete. The umbones are extensively eroded, and the valves are narrowed in the middle. It loves to lurk among the gravel and small stones in the shallows of quick-flowing rivers or mountain streams. It is found in the North of England, North and South Wales; near Ross, in the Wye; Devonshire and Cornwall; in the rivers flowing from the Scotch Highlands; and in many of the North and South Irish rivers.
It burrows its shell somewhat obliquely, a small portion of which is thus only exposed. A diligent search is required to find it, as by the growth of confervæ upon the little exposed portion, it cannot easily be distinguished from amongst the surrounding stones.
It is very susceptible to the action of light, · opening the valves on a hot sunny day; but if the
sun be overcast they remain closed. Country boys wade for them, or take them by thrusting the end of a long slender rod into the partially open shell, which closes upon it, and the prize is thus dragged to shore.
The Pearl Mussel enjoys a reputation as one of the few British bivalves which contain the beautiful production whose name this species bears. The other Unios and the Anodon occasionally yield pearls, as also the marine mussel (Mytilus edulis) and the oyster.
Pearls are of the same nature as the nacreous layer of the shell, and are abnormal secretions of the mantle, composed of alternate layers of animal membrane and calcareous matter, developed around some foreign body,-a grain of sand, a parasite, or an unfertilized ovum.
The great Linnæus owed in part his elevation to nobility to a discovery of causing this freshwater mussel to produce pearls at pleasure. This was accomplished, it is conjectured, by boring small holes through the shell and introducing a particle of sand, which would become a nucleus round which a pearl would be developed; but the artificial production of pearls had been long known to the Chinese. The Avicula margaritifera of the Indian seas is the most famous for pearls.
Pearls have been associated with the name of Britain from the very earliest known times. Suetonius gives as the reason for Cæsar's expedition into Britain, the search for pearls, which Pliny seems to confirm, saying that Cæsar gave a breastplate covered with British pearls to Venus Genitrix, and hung it in her temple at Rome; he further adds that they (probably from Mytilus edulis) were small and ill-coloured, and Tacitus says the same; but the Venerable Bede, on the other hand, states that the British pearls were excellent and of all colours—reddish, pale violet, and green. In an old translation of Boetius, by Bellenden (1541), the following allusion is made to British pearls :-"In the horse mussillis are generit perlis. Thir mussillis airlie in the morning, when the lift is clear and temperate, openis thair mouthis a little aboue the watter, and maist gredelie swellis the dew of heaven, and aftir the measure of the dew they swellie, they conceive and bredis the perle.” Camden, still later, in his “Britannica,” speaks of the shell-fish of the little river Irt, in Cumberland, " that they, by a kind of irregular motion, take in the dew and produce pearls.”
The Pearl Mussel was formerly an object of considerable fisheries in our own country, as it is now in some parts of Germany. So, also, the common mussel, a pearl fishery of which continued to exist up to a very recent period at the mouth of the river Conway, in North Wales. A patent was also granted early in the present century to fish for pearls at the mouth of the river Irt, in Cumberland. Higher up both rivers, however, the Unio has been at various times also fished to a great extent for the ornamental excretions to which it is subject. The pearls from Mytilus edulis are very much inferior in quality and size to those from the Unio. Those of the Conway had great fame. Extensive fisheries existed in the rivers of Tyrone, Derry, Donegal, near Dundalk, near Waterford, and in Kerry. In Scotland, the Tay was the seat of a pearl fishery. “It is said,” writes Captain Brown, “that the pearls sent from thence to London from the year 1761 to 1764 were worth £10,000, and it is not uncommon at the present day to find pearls in the Teith and Tay worth from £1 to £2 sterling each.” The var. Roissyi was formerly much sought for in the Black River, Kirk Braddam, Isle of Man, on account of its pearls.
ANODON CYGNEUS—(the Swan Mussel) (Pl. I., fig. 1)--attains a size considerably larger than the Unios. The maximum size exceeds in length eight and in breadth four and a half inches. It is readily distinguished by its rather thin, oval (truncated behind) shell, compressed in the young, but becoming ventricose with age. The epidermis is glossy, dull green, more or less tinged with dusky, and slightly radiated; the inside of the shell is bluish-white, pearly in young and yellowish-white in old shells. The umbones