[Kingdom] Animalia Linnaeus, 1758 - animals
   Grade Enterozoa
     Grade Cœlomata
       Phylum Mollusca Linnaeus, 1758 - molluscs
         Branch Eucephala
           Grade Echinoglossa
             Class Cephalopoda Cuvier, 1795 - cephalopods
               Grade Siphonopoda
                 Order Tetrabranchia
                   1 Genus Orthoceras Bruguière, 1789
                   2 Genus Nautilus Linnaeus, 1758