Life history of Ulnaria ulnabiseriata, from initial cell, via pre-normal cells, finally to normal cell, external view, SEM A initial frustule, note its cylindrical and twisted frustule (for details see Fig. 5) B–H pre-normal frustules/valves, note twisted frustules with smoothly tapering apices (B–E, H), protracted and sub-capitate to rostrate apices (F, G) I normal valve. Note: the number value at the top of each figure refers to the length of each specimen.