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 Hemiaulales Round & R.M. Crawford (1990) Family Streptothecaceae R.M. Crawford (1990) 1 Genus Streptothecaᵀ Shrubsole (1890) see Genus Helicotheca Ricard (1987) 2 Genus Neostreptotheca Stosch (1977)