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 (cf. Orthalicidae, although the marine gastropod family Pyramidellidae is larger). 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".