Page:Natural History, Mollusca.djvu/93

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81
LUNG-BREATHING MOLLUSKS.

dage is very minute, concealed within the substance of the mantle, or altogether wanting. Our own country furnishes examples of each of these kinds. All the native species are comprised in five families,— Cyclostomadæ, Auriculadæ, Limneadæ, Helicidæ, and Limacidæ.


Family Cyclostomadæ.

This is an extensive family, though represented in Britain by but a single species. The great majority of its members are elegantly formed and beautifully sculptured shells from warm countries. The shell is spiral, ample in its dimensions, with a circular aperture, generally surrounded in the adult with a frill-like, shelly expansion; it is closed by a spiral, shelly operculum.

The animal has a broad foot, divided longitudinally; a central spiral body, enveloped in a single-edged mantle.
THE ELEGANT CYCLOSTOMA.
There are two tentacles, which are lengthened and contractile, with an eye placed on the outer side of the base of each. The sexes are distinct. All the species are terrestrial, and are generally found on trees, on the leaves of which they feed. The only British species (Cyclostoma elegans) is a little shell, about half an inch in length, and rather less in width, of a grey or purplish yellow hue, often marked with two rows of dark spots. The spire is composed of five rounded whorls, marked with numerous close-set, raised lines, running spirally, with finer longitudinal ones between