Page:Natural History, Mollusca.djvu/215

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203
MARSH SNAILS.

laminæ ranged round a nucleus commonly placed in or near the centre.

Unlike the great majority of tlie Comb-gilled Mollusca, the members of this family are confined to fresh waters, inhabiting lakes, streams, and marshes. They are distributed all over the world; and occur in a fossil state as far back as the oolitic series. The fine globose shells which are found in the rivers of tropical countries, known as Apple-snails (Ampullaria), are but slightly separated from this family.

Genus Bithinia.

In addition to the family characters, those which distinguish this small group are, that the operculum has a thick shelly coat on the inner surface, and has the nucleus nearly central; and that the aperture of the shell has a slightly thickened rib, along the interior of the margin.


BITHINIA TENTACULATA,
(magnified).
Two species of this genus are found in the streams and ditches of this country, the more common of which is the Tentacled Bithinia (Bithinia tentaculata). It is about half an inch in length; the shell is often covered with a blackish foul coat; the spire is composed of five whorls, the lowest of which is swollen. The animal is purplish black, with brilliant yellow specks.

This genus, like the Mollusca generally, produces eggs, while its fellow-genus Paludina is viviparous. The mode in which the eggs are laid