Fam. 1. NERITIDE.

The sides of the body simple, without any elongated filaments; tentacles awl-shaped, eyes on short pedicels at the outer side of their base (p. 78. f. 1, 2.); the shell ovate-conical; mouth half-ovate, with a flattened transverse sharp-edged inner lip; the operculum spiral, half-ovate, and furnished with two internal processes on its front edge, forming a kind of hinge on the sharp edge of the inner lip of the shell. See p. 78. f. 3.

The peculiar structure of the operculum makes this family more closely resemble the bivalve shells: the processes appear to answer the same purpose (that of keeping the two parts in their proper situation) as the teeth of the hinges in the bivalves.

In the exotic genus Navicella, which, on account of its large mouth, has been confounded with the Patellæ, the processes occupy the greater part of the operculum.

There is only a single fluviatile genus of this family found in Britain.

1. 1. NERITINA Lam. (Neritine.)

Shell half-ovate, thin; inner lip slightly toothed; operculum only slightly calcareous, and furnished with a sharp flexible outer edge; foot short, rounded at each end. This genus is separated from the marine Nerita by the pillar being sharp, only slightly

denticulated, and the outer lip not being toothed within.

Neritina is the diminutive of Nerita, the ancient name of a sea-shell.

The greater part of the species are confined to freshwater streams, but one of the North American species is found for 200 miles up a river, from the mouth where it is quite salt, to beyond the reach of the tide, where the water is perfectly fresh. One species (Neritina viridis) is only found in the sea. 1835.)

(See Phil. Trans.

M. Deshayes and several other conchologists, especially those who only study the external form of shells, have proposed to unite this genus to the Nerites, because some of the species are marine, and some of the fluviatile species have a tooth on the pillar lip. The genera are, however, very distinct; and they may be well characterised by the structure of the operculum. (See Phil. Trans. 1833, p. 814.) The operculum of the Neritina is solid, shelly, and furnished with a thin flexible outer edge; that of the Nerites is horny, covered on both sides with a hard shelly coat. The position of the horny operculum is shown by a groove in the edge between the two coats; and if a knife is inserted, the coats can be separated from the operculum.

As the periostraca is essential to the structure of the shell, and is always present, some shells being formed of scarcely any thing else, so it is with the operculum, the horny part similar to the periostraca of shells being always present, and forming its essential part, and a shelly coat being in some instances added to the outer surface, as in Turbo and Phasianella, or to

the inner surface, as in this genus, in which the horny part is very thin and scarcely visible, except where the shelly coat is very thin, as at the flexible edge.

These animals absorb the septa which separate the whorls of the spire, when they have arrived at their full size, so as to allow more room for the spiral body, without increasing the size of the shell; and this can be done without endangering the strength of the shell, as only a very small part of the whorl is exposed on the surface. A similar absorption is to be observed in many Auriculidæ, and to a less extent in the Cones, where the septa are only reduced in thickness. (See Phil. Trans. 1833, p. 798.)

This absorption is only superficial, and produced by that portion of the surface of the mantle which lies close to it, and is not to be confounded with the absorption of the bones of vertebrated animals, where it is produced by vessels which ramify in the substance of the bone, and which are accompanied by other vessels to replace with new portions the part which has been removed.

The apices of the spires of these shells are sometimes eroded; those are more so which live in stagnant or nearly stagnant waters. The late Mr. Sowerby (Min. Conch. iv. 49.) supposed that this was produced by "some acid developed during the fermentation of vegetable matter in marshes or at the bottoms of the rivers." Others, who were not aware how the animals walked, have said that this erosion of the apex was produced by the animal rubbing it against the ground in progression; explaining also the erosion of the umbones of the Uniones in the same manner.

1. 1. NERITINA fluviatilis. River Neritine. (t. 8. f. 124.) Shell convex, dilated, tessellate, with variously coloured spots; spire short, lateral.

Neritina fluviatilis. Lamarck, vi. ii. p. 188.; Flem. B.A. 321.; Turton, Man. ed. 1. 138.

Nerita fluviatilis. Linn. S. Nat. 1253.; Müller, ii. 194.; Drap. p. 31. t. 1. f. 1—14.; Brard, p. 194. t. 7. f. 9, 10. 12.; Mont. p. 470.; Turt. Dict. 127. Theodoxus Lutetianus. De Montfort, ii. p. 351. Neritina fontinalis. Brard, Hist. C. 196. t. 7. f. 11. 13.; Pet. Gaz. t. 91. f. 3.; List. Conch. ii. 1.38.; Swamm. B. N. 80. t. 10. f. 2.

Neritina Dalmatica. Sow. C. Illus. f. 57.

In slow rivers, adhering to stones.

Animal white; head and back of the neck blackish; hinder part of the foot sometimes black spotted; tentacle long, white, with blackish line.

Shell about three eighths of an inch long, and two broad, convex above and flat underneath, obscurely striate transversely, of a greenish or whitish colour, variously checquered with spots or bands of white, brown, purple, or pink; spire consisting of three volutions, the first very large, oblong, and oblique, the others small and lateral; aperture horizontal, semielliptic, with the margin sharp and entire; pillar white, transverse, sloping down to a sharp edge, and quite entire; operculum semilunar, yellowish, with an orange border, and underneath is a strong raised grooved spire at one end.

The shells are often covered with calcareous incrustations, deposited by the water, which make them

look like pieces of dirt, and thus escape being seized on by the fish.

The continental conchologists have described several species allied to the above. Rossmäsler reduces them to three; but, from the specimens which I have received under different names, I greatly doubt if they are more than mere local varieties of our species. Nilson found a small variety or species in Sweden, on the shores of the Baltic, with Mytilus edulis, Cardium, &c. Our species has been found in similar situations in Loch Stennis, Orkney, by Mr. Edward Forbes.