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.
The Century Dictionary and Cyclopedia
An order of Cephalopoda, named by Owen from the two pairs of gill-plumes, or ctenidial branchiƦ.