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brought before it, the valves are at once closed, or partially so. On being introduced into the aquarium, it at once reconciles itself to its new abode, mooring itself by its threads to the stones, or the sides of the glass. Young individuals are harder to please, exhibiting their dissatisfaction by their peregrinations, making the circuit of the aquarium before selecting a resting-place; but on finding a suitable spot, they follow the example of their seniors and secrete a byssus, and there remain fixed for life.
The present species is now found in almost every part of Europe, in canals, tanks, running streams, and rivers, attaching itself by its byssus of strong threads to stones, the live shells of Anodon, and the dead ones of their own species, wood piles, or brickwork. In docks it luxuriates beneath the floating timber; in canals it abounds beneath the shadow of bridges. It has not yet found its way into Ireland.
Extinct species of Driessena inhabited the fresh waters of the Isle of Wight during the Upper Eocene epoch. D. polymorpha is, however, absent from the newer Tertiaries of this country,
This group contains several genera of exclusively fresh-water habits. There are only two European genera, which are also British,-Unio and Anodon, characterized by their oblong shells, the mantle-lobes free all round except at the posterior side, where the edge is bearded. Anodon is distinguished from Unio by the absence of teeth on the hinge-line; hence called edentulous.
The Unionido have all large shells; the animals bury themselves vertically in the mud of rivers, &c., the posterior side upwards. The exposed portion of the shell is usually encrusted with a calcareous deposit. The umbones, especially of Unio, are generally much eroded by the acids dissolved in the water. Unios are the most ancient of the fluviatile Mollusca. A Unio or Anodon appears in the Old Red Sandstone of Kilkenny. They characterize the fresh-water deposits of the Purbeck, Hastings Sands, Weald Clay, and Middle and Upper Eocene strata. A species, U. littoralis, now living in the north of France and Sicily, occurs in the fluviatile deposits at Clacton, Ilford, and Cropthorn. U. pictorum, U. tumidus, and Anodonta cygnea are found with it.
UNIO TUMIDUS—(the swollen Fresh-water Mussel) (Pl. II., fig. 2)—(Unio, Lat., a pearl).--The shell is about three inches long and one and a half wide, oval, solid with a thick glossy dark brown epidermis. The umbones are prominent; the lunule is lanceolate and narrow; the ligament is short, thick, and prominent; the anterior side is rounded and regularly sloping towards the front; the posterior side wedge-shaped. The interior is bluish-white or salmon-colour.
It inhabits canals and slow-running rivers with a muddy bottom, burying itself in a vertical position more than one-half of its length. It extends as far north as Yorkshire.
UNIO PICTORUM—(the Painters' Mussel) (Pl. II., fig. 3).--The specific name originated in the use of these shells for holding colours by Dutch painters. The shell is less solid and of a more oblong form, and necessarily of greater proportionate width than that of U. tumidus. The epidermis is thin and beautifully coloured, of a shining greenish-yellow, banded with brown. The length is about two or three inches. It is associated with the last species.
UNIO MARGARITIFERUS—(the Pearl or Black Mussel) (Pl. III., fig. 15).—This species, also