Description

Frustules araphid, symmetrical to apical and transapical axes, although some species are triradiate rather than bipolar. Axial area distinct, central area expanded bilaterally, unilaterally or absent. Genus distinguished from Synedra by specimens forming short to extremely long ribbon-like colonies. Members of the genus are sometimes difficult to recognize since their filamentous nature is not always apparent in cleaned material.

Labiate process and spines present, valves linear to elliptical, apices rostrate or capitate. At valve center, the striae are often occluded, appearing as ghost structures. Apical pore fields present.

The taxonomy of this genus has been debated over the last several year, between "splitters" (who recognized five additional genera in what have been classically recognized as Fragilaria species, including Fragilariforma [Fragilaria virescens and allies], Puncastriata [Fragilaria pinnata and allies], Pseudostaurosira [Fragilaria bevistriata], Staurosira [Fragilaria construens] and Staurosirella [Fragilaria lapponical], and "lumpers" (e.g. Krammer and Lange-Bertalot 1991) who combine most Synedra species as well as Opephora and Hannaea with Fragilaria. We continue to follow the classical approach until more convincing evidence is presented by either side of the argument.