Pre-normal frustule of Ulnaria ulnabiseriata, external view, SEM A pre-normal frustule, note its cylindrical and twisted outline B, C middle part details from A, note small central area flanked by striae on one side (B, two arrows), girdle bands lacking D, E apical details from A, note laterally located sternum (E, double-headed arrow) and flush ocellulimbus F other half from A, note laterally located sternum (double-headed arrow). Scale bars: 20 μm (A); 4 μm (B–F).