Kingdom Chlorobiota C.Jeffrey ex Doweld (2001) - plants see Subkingdom Viridiplantae Caval.-Sm. (1981)
   Subkingdom Pteridobiotina Britton & A. Br. (1896)
     Superdivision Cycadophytanae Doweld (2001) see Superphylum Cycadophytanae Doweld (2001)
       Division Cycadophyta Bessey (1907)
         Class †Cycadeoideopsida D.H.Scott (1923) [monotypic]
           Order †Cycadeoideales E.W.Berry (1916)
             Family †Williamsoniaceae Nath. (1913)
               1 Genus †Cycadocephalus Nath. (1902)
               2 Genus †Fredlindia J.M.Anderson & H.M.Anderson (2003)
               3 Genus †Ischnophyton Delev. & R.C. Hope (1976)
               4 Genus †Weltrichia Braun (1847)
               5 Genus †Williamsonia Carruth. (1870), nom. cons. prop.