Tet`ra*bran`chi*a"ta\, n. pl. [NL. See Tetra-, and Branchia.] (Zo["o]l.) An order of Cephalopoda having four gills. Among living species it includes only the pearly nautilus. Numerous genera and species are found in the fossil state, such as Ammonites, Baculites, Orthoceras, etc.