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Abstract
Detailed morphological observations at the light microscopy (LM) and scanning electron microscopy (SEM) levels were made on North American populations of Distrionella incognita (Reichardt) Williams (=Fragilaria incognita Reichardt). Results are used to show the distinctiveness of this taxon from closely related species and to justify its separation from Fragilaria Lyngbye as suggested by Williams (1990a). The protologue of Distrionella Williams is herein emended to include features newly discovered under SEM. Likewise, two recently described species and a form, Fragilaria germainii Reichardt and Lange-Bertalot, Fragilaria husvikensis Van de Vijver, Denys et Beyens, and Fragilaria germainii fo. acostata Lange-Bertalot are here transferred to Distrionella based on LM and SEM data presented by their original authors. Relationships among these taxa and other morphologically related species are discussed in this work. Ecological attributes are provided for habitats that support Distrionella incognita in North America.