nub. The aperture usually has visible folds.

The clausilium

Clausiliids are also very unusual among pulmonate gastropods in that most of them have a "door" or clausilium. The clausilium is not the same thing as an operculum, which does not exist at all in most pulmonate gastropods.

The clausilium is a calcareous structure, tongue-shaped or spoon-shaped, which can close the aperture of

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