Ulnaria constricta-biseriata sp. nov., external view, SEM A a complete valve B middle part from A showing the constricted middle margins and central area forming a rectangular hyaline region C, D two apical details from A, note a few serrated apical projections protruding over the ocellulimbus (two black arrows) E another middle part, note two constricted middle margins F another apex, note a few serrated apical projections protruding over the ocellulimbus (arrow) and the closing plates (wavy arrow). Scale bars: 20 μm (A); 2 μm (B–F).