Division Bacillariophyta Engl. & Gilg (1919) see Phylum Bacillariophyta Engl. & Gilg (1919)
   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 Biddulphiopsis Stosch & Simonsen (1984)
           3 Genus Hydrosera G.C. Wall. (1858)
           4 Genus Isthmia C.Agardh (1832)
           5 Genus Trigonium Cleve (1868)
           6 Genus Terpsinoë Ehrenb. (1843) see Genus Terpsinoe Ehrenb. (1843)
           7 Genus Pseudotriceratium Grunow (1884)