Page:Natural History, Mollusca.djvu/216

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is curious, and has been particularly described by M. Bouchard Chantreaux. According to this careful observer, the Tentacled Bithinia lays from May to August.

Family Neritidæ.


A single British species represents a family which, in warmer climates, plays a conspicuous part along the shores of the ocean, and in their rivers and lakes. The members have solid shells, more or less oval, the last whorl so greatly developed as to occupy by far the principal portion, the aperture very open, somewhat crescent-shaped, with an expanded and flattened inner lip.

The animals have broad muzzle-shaped heads, with awl-shaped tentacles, and eyes on short footstalks; the foot is somewhat three-sided, without any lateral filaments. An operculum is always found, which is spiral, semi-oval, and furnished with two internal processes on its front edge, forming a sort of hinge on the sharp edge of the inner lip of the shell. Dr. J. E. Gray thinks that this peculiar structure of the operculum "makes this family more closely resemble the bivalve shells: the processes appearing to answer the same purpose (that of keeping the two parts in their proper situation) as the teeth of the hinges in the bivalves."

Genus Neritina.

The shell in this genus is semi-oval, without any perforation; the inner lip is slightly toothed, sharp-edged; the surface is smooth, or striated, or spinous, covered with a skin.