BYSSUS, in botany: A genus of the 57th natural order, viz. Algae, belonging to the cryptogamia class of plants. It has a drown, or very fine uniform powder. The character is taken from this circumstance, that they are covered with a simple capillary filament or down, resembling soft dust. There are 15 species, all natives of Britain, growing upon rotten wood, old walls, &c.