Eunotia metamonodon

Valves are arcuate. The ventral margin of the valve is concave, while the dorsal margin is convex and broadly arched about the center of the valve. Apices are broadly rounded, with a weak point at the termination. A slight constriction is present between the apices and the dorsal swelling of the main body. Helictoglossae are visible near the apices, at the dorsal margin. The raphe curves around the apices onto the ventral margin. Striae are interrupted by the sternum near the ventral margin. The striae are variable in their spacing, from regularly spaced to irregularly spaced. Areolae are distinct and number 23–27 in 10 µm.

Originally, E. monodon was described based on syntypes from two separate locations in Mexico, Guadaloupe and San Miguel (Ehrenberg 1843). An iconolectotype of E. monodon was designated (Rumrich et al. 2000) based on the Guadaloupe illustration (Ehrenberg 1843 Tafel II, Fig. V: 7). In common usage, many authors have used the name “E. monodon” to refer to a taxon that was greater in length and width than the Guadaloupe iconolectotype (Lange-Bertalot 2011). Lange-Bertalot recognized the discrepancy between the Ehrenburg illustration and the commonly reported taxon. In order to correct the discrepancy, Lange-Bertalot described E. metamonodon, a taxon based on specimens from Germany. This taxon accommodates the concept of “E. monodon” as used by many authors including Patrick and Reimer (1966), Foged (1981), Siver et al. (2005), Siver and Hamilton (2011), and Reavie and Kireta (2015). The size difference between the Guadaloupe iconotype for E. monodon and E. metamonodon was again made explicit in a recent volume (Hoffman et al. 2013).

Patrick and Reimer (1966) report narrow specimens (< 10 µm) of E. monodon. Their illustration (Pl. 11, Fig. 6) , however, conforms to the concept of E. metamonodon. Further investigation is needed to confirm whether E. monodon occurs in North America.