Initial frustule of Ulnaria ulnabiseriata, external view, SEM A Initial frustule, note its arcuate, cylindrical, and twisted outline B, C details of middle part from A, note small central area flanked by striae on one side (C, arrow), mixed striae, and underdeveloped sternum (i.e., striae continue across valve surface) D apical detail from A, note mantle not well differentiated from valve face (double-headed arrow), and virgae and vimines/viminules occurring nearly on the same plane E, F other apical details from A, note laterally located sternum present only near one apex (F, double-headed arrow), ocellulimbus flush with and extending onto valve surface, and its partial pervalvar rows of porelli not perpendicular to the valve plane (F, arrow). Scale bars: 20 μm (A); 2 μm (B–F).