Clausiliidae, also known by their common name the door snails, are a taxonomic family of small, very elongate, mostly left-handed, air-breathing land snails, sinistral terrestrial pulmonate gastropod mollusks. With about 1,300 species recent and fossil, this belong among the most diverse families of land gastropods.

Most species of Clausiliidae have an anatomical structure known as a clausilium, which enables the snail to close off the aperture of the shell with a sliding “door”. Almost all the species of snails in the family of door snails are left-handed, which is an uncommon feature in gastropod shells in general.

These snails have shells which are extremely high-spired, with numerous whorls. The shells tend to be club-shaped, tapering at both ends to a rounded nub. The aperture usually has visible folds. 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 pulmonate gastropods. The clausilium is a calcareous structure, tongue-shaped or spoon-shaped, which can close the aperture of the snail shell to protect the soft parts against predation by animals such as carnivorous beetle larvae. The narrow end of the clausilium slides in the grooves that are formed by the folds on the inside of the shell.