Eunotia formica

Valves are weakly arcuate. The dorsal margins are convex and the ventral margins are concave. Most specimens have gibbous swellings observed in the valve center. Apices are weakly capitate, with acutely rounded to cuneate ends. Helictoglossae are present near the apices. Terminal raphe fissures are visible on the valve face, around the apices to the dorsal margin. Striae are perpendicular to the valve margins. Striae are typically regularly spaced, although irregular spacing is common. A sternum is visible near the ventral margin. Areolae are distinct and number 23–28 in 10 µm.

The concept of E. formica presented conforms to the taxon in a broad sense (sensu lato). Lange-Bertalot et al. (2011) draw distinction within European specimens of E. formica, and recognize the species E. formicina and E. myrmica. These species were distinguished based on differences in apicies; rounded in E. formicina and acuminate in E. myrmica. Our specimens overlap in areola density described for E. formica sensu stricto, E. formica sensu lato, E. formicina, and E. myrmica, and show a range of apices from rounded to acuminate. Because we see such notable variation in a single population, we consider the variation to be attributable to phenotypic plasticity within E. formica.