Eunotia papilioforma

Valves are asymmetric to the apical and transapical axes, with one apex wider than the other. The ventral margin is straight to weakly concave, and the dorsal margin is convex. The apices are rounded, tapered and slightly delimited by a change in the slope of the dorsal margin. These characteristics of the apices, combined with the inset position of the helictoglossa on the ventral margin of each apex, give the apices a 'nose-like' appearance. Helictoglossae are set in at different distances from each apex (asymmetry to the transapical axis). One apical rimoportula is present at the wider apex, at the center of that apex. The distal raphe ends curve onto the valve surface. The striae are slightly radiate. Areolae approximately 50 in 10 µm.

Specimens of E. papilioforma are variable in the width of the valve apices and the relative position of the helicotoglossae. Some of this variation is a result of the plane of focus in the microscope; optical dissection is critical to observe valve morphology. Some specimens of E. papilioforma have one more attenuated apex and the helictoglossa is more removed, relative to the opposite end. The shape of the apex with the inset helictoglossa is more characteristic of E. incisa. These valves, however, are dissimilar to the overall E. incisa morphology, which lacks a strong heteropolar variation in apex width and position of the helictoglossae. Specimens of E. papilioforma are similar in morphology to the more asymmetric valves of E. rhomboidea. Eunotia papilioforma, however, is consistently wider than E. rhomboidea (as shown by Camburn et al. 1978; Krammer & Lange-Bertalot 1991). Figures 1-12 (Wuthrich 1975) were classified as E. rhomboidea but are likely more than one taxon. These illustrations show some of the variability discussed here, including variation in the width of the valve apices, position of the helicotoglossae and valve width.

In some specimens of E. rhomboidea, one apex is rounded and the other end is more shaped like the “snout of a shark”. Some specimens possess a small pseudoseptum. The pseudoseptum prevents observation of the base of the rimoportula (in SEM). The rimoportula of E. papilioforma also consistently occurS on the wider end of the valve.