are very small and convex; the lunule is indistinct, and the hinge-line is without teeth.

The Swan Mussels live in lakes, ponds, canals, and muddy rivers, and are found in suitable localities throughout the kingdom.

Anodons vary very much, not only according to locality, but in the same waters; some of the numerous varieties should, it is thought, be more justly regarded as species. In ponds where there is plenty of food, and where the water is nearly stagnant, they become of large size, with ventricose thin shells, and are type forms; whilst in more rapid rivers, with pure clear water, with very little decomposing animal or vegetable matter, they are small and comparatively longer than A. cygneus, with compressed thick shells, and are the A. anatinus (Pl. II., fig. 4) of some authors; but all intermediate forms and sizes may be observed.

The manner of locomotion is slow and regular, leaving their tracks distinctly discernible in the soft mud. At Bottesford, on the Trent, where at high tides the water is salt, it is found in great abundance.

Anodons are thrown up in quantities on the shores of Lough Schur, co. Leitrim, where they are eaten by the peasantry. Sliggaun is the common name applied to the Swan Mussel in the North of Ireland.

Anodons furnish a very favourite repast for the herons, and crows feed upon them. Pennant says, "that when the shell is too hard for their bills, they will fly with it to a great height, drop the shell on a rock, and pick out the meat when the shell is fractured by the fall.”

This species is invested with a mite (Atax ypsilophora), which is so tenacious of life that they survive an immersion in boiling water.

FAMILY CYCLADIDE.

The remaining British fluviatile bivalves belong to the family Cycladidæ, characterized by the mantle-lobes being united at the posterior side to form one or two prominent contractile respiratory siphons. The foot is large and tongue-like. The shells are small, thin, and suborbicular; the hinge-line with minute cardinal

Fig. 3.—Pisidium amnicum, with its foot and siphon protruded.

and lateral teeth, two cardinals, and a lateral on each side, in the left valve; in the right, one cardinal, and two laterals on each side.

The family contains two British genera, Cyclas and Pisidium; the respiratory siphons are two in number in Cyclas (Pl. III., fig. 17); there is only one (fig. 3, s) in Pisidium, the branchial and pedal orifices being confluent (b). The shells of Pisidium are inequilateral, those of Cyclas rounded, and more or less equilateral.

GENUS CYCLAS.

There are five British species of this genus, the largest of which is

CYCLAS RIVICOLA-(the River Cyclas) (Pl. III., fig. 17).-The shell of this species is distinguished by its great size, its more oval form, the strong concentric ridges on the shell, and the prominent ligament. The oval equilateral shell is ten lines in length, seven in width, and five in thickness; the epidermis is thick, of a glossy reddish or greenish-brown, with two or three darker bands, strongly striated concentrically; the interior of the valves is whitish, with a bluish or yellowish tinge.

This species burrows in the mud of canals and slow-running rivers, in the southern and midland counties of England: it is plentiful in the docks and canals about London; it occurs in the Thames, above Chelsea; in the Medway at Maidstone; Kennet and Avon Canal, Wilts; Severn, at Wainlode; Tewkesbury; Newent,

D

Berkeley, and Combe Hill Canals, Gloucestershire; Oxford; Chester; York; Doncaster; &c. It is a species of the newer Tertiaries.

CYCLAS CORNEA-(the Horny Cyclas) (Pl. III., fig. 18).—The shell of this species is not more than half the size of the last, and more globular; its length about six lines, breadth four, and thickness three and a half; equilateral, finely striated concentrically, of a yellowish-brown colour, with paler bands; ligament indistinct externally.

It varies very considerably: there are three well-known varieties.-1. var. flavescens (Pl. III., fig. 16), with a smaller and rounder shell, the body and shell straw- or lemon-coloured. 2. Which is probably the fry of C. cornea, with a small, nearly globular shell. 3. Gibbous at the beaks, but thin or compressed towards the edges.

Few pools, ditches, or streams throughout the United Kingdom are without this very interesting bivalve. In summer it is found among the confervæ floating near the surface of the water; in winter it buries itself in the mud.

No aquarium can be considered at all complete without this interesting animal, whose habits are so much at variance with the popular notions regarding the way in which mussels, cockles, and other bivalves pass their days. It crawls readily by the aid of its long foot, or ascends the sides of the glass, and, on reaching the surface, moors