An order of the true mosses (subclass Bryidae), consisting of a single species, Schistostega pennata, the cave moss, which is especially characterized by its leaf arrangement and the form of its protonema. The plants grow in dimly lit, cavelike places, such as the undersides of upturned tree roots. The cave moss is widely distributed in north temperate regions. An unrelated moss in Australia and New Zealand, Mittenia plumula, has an identical protonema and habitat.