An order of Cephalopoda, named by Owen from the two pairs of gill-plumes, or ctenidial branchiæ.
the GNU version of the Collaborative International Dictionary of English
noun plural.
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.