Life history of Ulnaria ulnabiseriata, from initial cell, via pre-normal cells, finally to normal cell, ×400, LM A initial frustule showing its cylindrical and twisted outline, central area flanked by striae on one side B–N pre-normal frustules, note twisted frustules with laterally located sterna (B–E), one apex twisted and with almost centrally located sterna (F, G), almost centrally located sterna and complete central areas (H–N). O Normal valve.