Ulnaria sangzhi-biseriata sp. nov., external view, SEM A a complete valve B middle detail from A, note rectangular central area C, D two apical details from A, note a few serrated projections protruding over the ocellulimbus (arrows) E another middle part showing rectangular central area with ghost striae F another apical detail, note a few serrated projections (two arrows) and the closing plates (two curved arrows). Scale bars: 10 μm (A); 2 μm (B, E); 1 μm (C, D, F).