Kingdom Protoctista Hogg, 1860
   Phylum Bacillariophyta Engl. & Gilg (1919) - diatoms see [unranked] Diatomea Kütz. (1844)
     Class Coscinodiscophyceae Round & R.M. Crawford (1990) - radial centrics
       Subclass Biddulphiophycidae Round & R.M. Crawford (1990) see Class Mediophyceae Medlin & Kaczmarska (2004)
         Order Biddulphiales Willi Krieg. (1954)
           Family Biddulphiaceae Kütz. (1844)
             1 Genus Biddulphia Gray (1821)
             2 Genus Hydrosera G.C. Wall. (1858)
             3 Genus Isthmia C.Agardh (1832)
             4 Genus Trigonium Cleve (1868)
             5 Genus Terpsinoe Ehrenb. (1843)