Eunotia macroglossa

Valves with the ventral margin slightly concave to nearly straight, the dorsal margin evenly convex up to the rounded ends, with apices slightly set off by a change in slope of the dorsal margin, 21 – 93 μm long, 4.5 – 6.6 μm wide. Helictoglossae prominently visible in the light microscope as apical thickenings. Raphe extending from helictoglossa to 20-30% of the length of the valve. Striae slightly radiate, radiate at apices, 11 – 14 in 10 μm in the valve center, 16 – 18 in 10 μm near the valve apices. Rimoportula apical, at one end of valve, nearly parallel to striae.

In valve outline, it is most similar to E. minor, and bears particularly a close similarity to a valve out of the Taunus Mountains in Germany (Krammer & Lange-Bertalot 1991, Tafel 142, Fig. 15) attributed to E. minor. Although this previously figured valve may be conspecific with E. macroglossa, the size range of specimens from the type locality includes valves of much greater length than reported for E. minor, and the ends are not nearly as attenuated as in that taxon (Krammer & Lange-Bertalot 1991, Tafel 142, Figs 7-9, 13, 14). Eunotia macroglossa keyed to E. soleirolii based on the European flora (Krammer & Lange-Bertalot 2000); however it differed markedly in overall valve shape. Similarly, E. macroglossa did not fit the description for E. pectinalis with which E. soleirolii is sometimes confused. Eunotia soleirolii and E. pectinalis have parallel edges that end in a slight shoulder at the valve edge (Krammer & Lange-Bertalot 1991), in contrast with E. macroglossa which had a slightly curved dorsal edge, with the valve ends only very slightly set off from the evenly curved dorsal margin. The helictoglossae of E. macroglossa appeared further from the apices than its position in both E. soleirolii and E. pectinalis. Additionally, SEM micrographs of E. soleirolii show spines on the valve apices at the junction of the valve face and mantle, which were absent from E. macroglossa. Other distinguishing features of this taxon include the radiate striae at the valve apices. These are clearly visible even in most LM views (if ends are focused for this feature), and are visible in external SEM’s of all valves. The external pore of the rimoportula is inconspicuous in comparison to many taxa. The mantle has a large hyaline area associated with the raphe as it curves from the mantle up on to the valve face. The raphe end on the valve face is usually simply curved, but can have a more irregular structure.