Page images
PDF
EPUB

eighths of an inch long, by a quarter broad, composed of three whorls, the last one excessively disproportionate; the spire very short, and as if lateral. A polished brownish-green horny epidermis covers the shell; in dead shells the beautiful spots or bands of white purplishbrown or pale brown are seen to perfection, for in fresh shells this colouring is much hidden by the epidermis, but may be seen by the aid of a magnifying-glass.

The species is common in all the larger rivers where the bottom is stony, and in canals upon stones. It does not occur in the north of Ireland, and is rare in Scotland, but is recorded from Loch Stennis, in the Orkneys.

The animal is rarely to be seen in motion: it crawls slowly, with the shell slightly raised; the long, slender divergent tentacles, the large black eyes, placed on short stalks at the base of the tentacles, are then shown; the skin is covered with stiff silky hairs.

The lingual teeth (see fig. 9):-the central tooth (a) is minute; the first lateral tooth is

a

Fig. 9.-Teeth of Neritina fluviatilis (Lovèn).

large, subtriangular, succeeded by two very minute ones; the uncini are about sixty in number; the first one is very large, and of a remarkable shape; the rest are very slender, hooked, and denticulated.

Each female deposits her egg-capsules in. clusters of fifty or sixty, on the surface of the shell of her neighbour and not to her own; sometimes, but rarely, to stones, or to the shells of other mollusks. Each capsule contains from forty to sixty eggs, but only one embryo is developed; for the other eggs constitute the food of the young Neritina until it quits the capsule. The capsules, which are usually mistaken for the eggs, are globular, and consist of two separable portions; the upper and larger falls off when the young is about to escape; the lower portion remains attached to the surface of the foster-parent's shell, the raised margins of which produce small indentations.-Claperède.

and

The young appear in August and September, creep about on the shells or stones which bore their egg-capsules, feeding on the microscopic organisms, diatoms, algæ, &c., which now serve for its nourishment.

FAMILY LITTORINIDÆ.

The three following species have little claim to be regarded as fresh-water shells; the above

family, to which they belong, contains the periwinkles and numerous other marine snails. The family may be distinguished from the preceding ones by the shell being spiral and conical, the operculum spiral, and the eyes sessile at the outer bases of the tentacles.

The genus ASSIMINIA (from the Latin assimilis, very like) is represented by one British species, ASSIMINIA GRAYANA (Pl. II., fig. 8), inhabiting the banks of the river Thames between Greenwich and Gravesend, living on the mud beneath the shade afforded by Scirpus maritimus, Festuca arundinacea, &c. It is very abundant.

The animal of Assiminia differs from the marine. Rissoa, in the tentacles being united to the eye-stalks, which equal them in length. The shell of A. Grayana is ovate, acute, solid, shining, of a liver-brown colour, and is about

Fig. 10.-Teeth of A. Grayana (Lovèn).

a quarter of an inch in length; the whorls are five in number; the suture is slightly impressed; there is no umbilicus; the aperture is ovate; the operculum is horny, ovate, and of a

black-brown colour. The teeth are seven in number; the central with a base produced into a horn, with five to seven pointed lobes; the first lateral tooth with seven lobes; the second is slender, claw-like, and serrated; the third is rounded at the tip, with minute denticulations. Lovèn.

In company with the last species are two other brackish-water shells, belonging to the allied genus HYDROBIA, species of which are more particularly marine. Hydrobia differs from Assiminia in having the eyes placed on tubercles, and from Rissoa in its smooth shell.

The form of the shell of one species, HYDROBIA SIMILIS (Pl. II., fig. 7), resembles that of Bithinia Leachii, but is distinguished by its smaller size and grooved suture; the operculum is horny, concentric, with the nucleus lateral; whereas in Bithinia it is somewhat shelly, and marked by concentric ridges having the nucleus central.

A second species, HYDROBIA VENTROSA, is closely allied to H. ulva, whose habits are more marine, from which it differs in being half the size, with a deeper suture; from H. similis in its longer spire, and the absence of the channelled suture, and by its much smaller umbilicus. This latter species is very abundant on many parts of our coasts, in estuaries and in brackish water, or upon the mud banks of tidal rivers.

[blocks in formation]

THIS

slugs, and such of the water snails which breathe air: they are closely related to Paludina, and to the plant-eating sea snails. Their breathing organ is the simplest form of lung, formed by the folding of the mantle, over which the bloodvessels are distributed, and occupies the same position as the branchial chamber of Paludinu, &c. This chamber.opens externally by a round contractile aperture on the margin of the mantle over the neck, on the right side. There is no special mechanism of respiration. The functions of both sexes are united in each individual. The Pulmonifera vary much in appearance and habits. The form and number of teeth on the lingual ribbon afford good characters for the distinction of genera and families, as also for the separation of allied species. The class is divided into two orders,―Inoperculata, without an operculum, and Operculata, possessing an operculum.

F

« PreviousContinue »