Eunotia tetraodon

Valves are highly arched. The dorsal margin has a constant 4 bluntly pointed undulations. Terminal undulations are distinct from and don't coalesce into the apices. The ventral margin is concave and follows the same curvature as the dorsal margin. The apices are wider than the undulations and obtusely rounded. Distal raphe ends lie along the valve face very close and parallel to each end. Striae are radiate throughout and of two types: complete striae that extend the full width of the valve and short striae that occur only along the dorsal margin. Short striae are more numerous in the undulations where they fill the gaps between complete striae. A fine transparent line runs close and parallel to the ventral margin. Striae on either side of this line may be continuous or displaced. Areolae in the striae are easily resolved in LM and number 20-25 in 10 µm.

Two morphotypes are present in the United States: The morphotype illustrated here (Morphotype I) and one in which the two central undulations are broadly rounded and the two outer undulations are acutely rounded (Morphotype II; see plate 10, figs. 6-10 in Werum & Lange-Bertalot 2004 and plate 105, figs. 5-9 in Lange-Bertalot et al. 2011).