Details of late pre-normal frustule of Ulnaria ulnabiseriata from Fig. 4D, external view, SEM A middle part detail, note rectangular central area B half valve, note its arcuate outline C one apical detail, note biseriate striae developed at apex D, E other half part of valve, note slightly twisted frustule and mantle joining the valve face at a right angle F apical detail from D, note inset ocellulimbus, developed closing plates (two wavy arrows), and few serrated apical projections protruding over the ocellulimbus (three black arrows and two white arrows). Scale bars: 6 μm (A–E); 3 μm (F).