\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.