Revision of the genus Stauroforma (Staurosiraceae, Bacillariophyceae) based on varieties of Fragilaria virescens described by Albert Grunow

A revision of the genus <i>Stauroforma</i> is proposed based on the analysis of several historic slides prepared by Grunow to describe <i>Fragilaria virescens</i> var. <i>exigua</i>. The original materials are analysed using both light and scanning electron microscopy together with a critical examination of the original drawings and notes Grunow made on his new taxa. Based on this, the genus description of <i>Stauroforma</i> is emended and the morphological descriptions of <i>S</i>.<i> exiguiformis</i> and <i>S. inermis</i> are updated. Grunow's variety <i>subsalina</i> is raised to species level as <i>Stauroforma subsalina</i>. A new species, <i>Stauroforma subcassubica</i> is described splitting it from <i>S. cassubica, </i>the latter having larger valves. The results of this revision will enable a more exact identification of the different <i>Stauroforma</i> species observed in freshwater and brackish water bodies in Europe.