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)
               Genus †Ischnophyton Delev. & R.C. Hope (1976)