Genus Unio.
The condition of the hinge affords the distinctive character in this family. The genus Unio has it furnished with a short crested anterior tooth, and a lengthened posterior lateral one in the right valve, shutting between two similar teeth in the opposite.
The shells of this genus, which are frequently large and richly pearly in the interior, are frequently used by artists for containing their colours. One of our British species is hence named Unio pictorum, or the Painter's Unio. Many of them yield pearls, which are generally superior to those of the marine Mussels. Our finest native species, hence named U. margaritiferus, is one of these. It is about five inches long, and half as broad, somewhat kidney-shaped, covered with a black epidermis, usually worn away at the beaks; the interior is pearly, but not brilliant. This species inhabits the rapid streams of mountainous districts in various parts of the British Islands.
"The Pearl Mussel, as this mollusk is familiarly called, enjoys a distinguished reputation as one of the few indigenous bivalves which yield the beautiful productions whose name it bears. In ancient times Britain enjoyed some celebrity for its pearls, as they constituted one of its attractions for Julius Cæsar, who, however, does not seem to have reaped a very rich harvest, so far as quality went, though he obtained enough in quantity to cover with them a buckler, which he dedicated to Venus Genetrix, and suspended in her temple. The pearls used for the construction of his present were probably such as Roman ladies would have scorned to wear,
