are at first exceedingly unlike the parent; they are provided with a delicate nautilus-like shell, and closed by an operculum; on each side of the head there extends a fin-like membrane, the edges of which are fringed with cilia, by means of which they swim within the contents of the egg. In most of the oviparous snails, on the rupture of the egg-cases, the young swim forth with great activity by the action of the lobes, and are dispersed far and wide.

Habits, Food, &c.-Paludina is a sluggish animal; it feeds on vegetable matter, and prefers nearly stagnant waters, or very slow-running rivers with a bottom of soft mud; it buries itself for weeks in the mud, and crawls up at intervals.

II. DESCRIPTION OF THE SPECIES.

The fresh-water Gasteropods of Great Britain are grouped in the families Paludinida, Littorinidae, and Neritinida.

FAMILY PALUDINIDE (Marsh Shells).

This family contains a few genera of fresh-water snails distributed in all parts of the world. In Great Britain three generic groups occur: Paludina, the type of the family; Bithinia,

and Valvata. Paludina is ovoviviparous, Bithinia and Valvata are oviparous; the operculum of Paludina and Valvata is horny, of Bithinia shelly. Valvata is distinguished from the others by the presence of a long plume-like gill, which is partially protruded when the animal is crawling, and by the more circular whorls composing the shell.

The lingual ribbon of Paludinida is very simple, and presents the same general features throughout the family, and places it in close proximity to the periwinkles and others comprised in the family Littorinida.

The general formula is 3. 1.3; the number of transverse rows is very limited.

GENUS PALUDINA.

PALUDINA VIVIPARA-(the Common Marsh Shell) (Pl. IV., fig. 26).—The specific name vivipara was given to this species by Linnæus, from the animal being ovoviviparous, that is to say, the eggs are retained within the interior of the parent until they are hatched; it is a peculiarity possessed by other species of the genus. As the next species very closely resembles the present one, it will be necessary to give a full description of it.

The shell is conically oval, rather solid, of rather dull yellowish-green, with three broad

Vincent Brooks, Imp

brown spiral bands on the last whorl, and two upon each of the two preceding whorls; the shell is also finely striated longitudinally; whorls six, rather convex, with a well-defined suture; aperture oval, peristome continuous, umbilicus represented by a narrow slit behind the inner lip; the operculum horny, rather thick. Length an inch and a half, and one inch broad. The body is a dark grey, or brown speckled with yellow. The young shells are sometimes, at least, clothed with a downy epidermis, rising into short spines round the middle of each whorl,-disappearing much on dying.

In the timber docks on the Thames, and in the various canals about London, this species is very abundant, and in fine condition. Following the ramifications of our system of water-roads, the species inhabits the Kent and Avon canals, the canals of Gloucestershire, Staffordshire, &c. ; but is by no means restricted to such habitats, for it is found in many of the slow rivers in the midland and southern counties of England. It occurs in the river Ouse and Barnsley Canal, at Wakefield, in Yorkshire, which are probably its northern limits. Dead shells of this species and of some southern forms are frequently introduced with the ballast into localities where it is not indigenous.

M. Joly, after keeping several individuals of