attached to the tails of fish, floating wood, &c. We see in this a provision for the dispersion of these sedentary bivalves. Nothing is known of the history of the embryo from this stage until it assumes the form of the adult.

As the Swan Mussel (Anodon cygneus) may be more within the reach of many of my readers, I would inform them that it does not possess a distinct lunule nor inter-locking teeth, nor are the muscular impressions so distinct; but in all other respects it resembles Unio.

The Food of bivalves consists of infusoria, diatoms, and vegetable matter, brought to the mouth by the portion of the branchial current, which is projected in that direction.

Collecting, &c.-The majority of the Conchifera live buried vertically in the mud: they may be collected by dredging with a kind of perforated tin saucepan, about six inches across, and furnished with a hollow handle, so as to receive the end of a stout walking-stick.

After making a scoop with the tin, it should be shaken, keeping the mouth just above the surface of the water, for fear of washing out any of the shelly contents; by this means the mud and fine sand pass out through the perforations of the tin, and the shells and gravel are retained. The large shells may be bagged, the smaller ones placed in glass bottles or tin boxes. To prepare

them for the cabinet, the shells should be first cleaned with a brush, and after being immersed in boiling water, the animals may be removed. The interiors of the shells should be wiped dry, and the valves closed, and tied with a moist tape or cotton; they should now be allowed to dry slowly, for if heated too much they are apt to crack.

II. DESCRIPTION OF SPECIES.

THE fresh-water bivalves of Great Britain are comprised in the three families-Mytilidæ, Unionidee, and Cycladida.

FAMILY MYTILIDE (MUSSELS).

This family is typified by the common marine mussel (Mytilus edulis) of our coasts, and is represented in our fresh waters by

DRIESSENA POLYMORPHA (the Zebra Mussel) (Pl. IV., fig. 25).—This bivalve closely resembles the common mussel; the shell is equi-valve, wedge-shaped, rounded behind; the umbones are placed at the anterior end; the valves are sharply keeled in the middle. The principal differences between this genus and Mytilus are as follows:-In Mytilus the mantle is open, in Driessena closed; in Mytilus the gills adhere through their whole length, in Driessena the extremities are free. In Driessena the anterior

adductor muscle is supported on a triangular shelf below the beak.

The mantle of Driessena is united all round, with the exception of three apertures (fig. 2)-one the anal orifice, which is prolonged into a very small tube; a second, the branchial orifice, furnished with a prominent siphon or tube, which is fringed on the inside.

The Zebra Mussel is an attached bivalve, like its brethren of the sea; the foot (f) is very imperfectly developed, giving

place to a gland, which secretes the material of the threads with which it attaches itself to stones, timber, and shells ; these threads are termed the byssus (b); and the third opening in the united mantle lobes is for the passage of these mooring cables. The epidermis of the shell is yellowishbrown, with undulating streaks, or zebra-like markings of dark brown.

This mussel was first discovered by Pallas, in the different rivers of Russia and also in the Caspian Sea; and from the great variety of forms presented by this species, he designated it under the name Mytilus polymorphus. Van Beneden, in 1835, showed it to differ from Mytilus, and constituted for it the genus Driessena, de

rived from the name of M. Driessenus, an apothecary of Mazeyk, from whom the former received in the year 1822 a collection of these mussels alive, from a canal near Maestricht. But as early as 1824 Mr. J. de C. Sowerby called attention to its occurrence in the Commercial Docks on the Thames, where it was already. abundant, and used by anglers as a bait for perch, whither it had been brought attached to timber from Eastern Europe.

The mode by which it has been introduced is evidently by its being affixed to the logs of timber before they were stowed in the ship's hold, for it has been seen adhering to them before they were unloaded, and not that it had attached itself to the ship's bottom, and so been conveyed. The former mode of transport is the more rational, as the bivalve can survive a removal from the water for several weeks, especially under such favourable conditions as prevail in the moist hold of a ship.

In 1833 it was found in vast abundance in the Clyde and Forth Canal, Glasgow; in 1834 it appeared in the Union Canal, Edinburgh; and in 1836 it was found in considerable numbers on the piers of the bridge which crosses the Nen at Fotheringay, in which locality it had been introduced from Wisbeach, on timber, since 1828. In 1837, the late Mr. Hugh Strickland found it

completely established on the beds of gravel in the river Avon, at Evesham, and also in the canal between Warwick and Birmingham, and in the canals near Wednesbury, in Staffordshire. He remarked that, as its propagation was so astonishingly rapid, it would become in a few years one of our commonest British shells. This has proved so true, that not only has it found its way throughout England, literally paving with its shells the beds and sides of our navigable rivers and canals, but it has even taken up its quarters in the water-pipes of London, Manchester, &c.

The Zebra Mussel made its appearance in the neighbourhood of Gloucester a few years after the opening of the Gloucester and Berkeley Canal, and has increased in numbers to such an extent, that it may be said to line the banks from the edge of the water to a considerable depth, throughout its entire length of sixteen miles. It appears in every available inch of space, from the water-line to the depth of fifteen or sixteen feet, upon the dock walls at Gloucester.

It is very tenacious of life and exceedingly prolific, provided the locality is favourable.

This mollusk is evidently sensible to light, which it would usually avoid, as exemplified in its occurrence in such prodigious numbers in water-pipes. If when the animal is at rest, with the shell partly open, an object is suddenly