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PULMONIFERA.—LIMNEADÆ.

of the fluid necessary for their respiration and life."

Genus Limneus.

In this genus the shell is ovate, oblong, or turreted, thin and horny, and transparent. The aperture is ovate or expanded, the margin entire, and the pillar marked with a single oblique plait running into the axis.
LAKE MUD SHELL. The animal has a short broad foot; the head and snout, and the tentacles, are also all characterised by peculiar breadth; the two eyes are placed in front of the base of the last-named organs. The mantle has an even edge sometimes reflected upon the shell, but never covering it. The tongue is armed with many transverse rows of short, hooked teeth.

The largest British species is the Lake Mud Shell (Limneus stagnalis), which attains a length of two inches, and a diameter of one. In the large rivers of Eastern Europe it grows to a much larger size. The shell is thin and brittle, of a greyish hue, often covered with an extraneous coat; the spire is composed of about seven whorls, tapering to a fine point; the last whorl is striated lengthwise,