Epithemia alpestris

Valves are dorsiventral with protracted and slightly reflexed ends. The dorsal margin of the valve is convex and the ventral margin is slightly concave. The costae are slightly radiate with an average of 2-3 costae in 10 µm. The striae are coarse, 10-12.5 in 10 µm, and have distinct areolae. There are often 4 striae between costae, but, due to the irregular spread of the costae, there can be a range of 2-6 striae between costae. The proximal raphe ends curve to a point that ends closer to the dorsal side than to the ventral side. At the central arch of the raphe, the top edge of the raphe terminates 2.8-4.6 µm from the dorsal edge, and the bottom edge of the raphe is 5.6-7.6 µm from the ventral side. The distal raphe ends are dorsally curved and terminate near the center of the valve apices. Between the proximal and distal areas, the raphe lies visibly along the ventral margin. Among individuals of the same species, there is some variation in the width of the costae, amount of reflection in the apices, and relative length to width ratio of the valves (Length:width in the studied populations was 3.7-6.5).

The treatment of E. argus var. alpestris in Patrick and Reimer (1975), differs significantly from Smith’s original image and description. The figure in Patrick and Reimer (1975) has a less arched dorsal margin, more rostrate and less reflexed apices, and much less apparent distal raphe ends. The populations observed in this study align more with Smith’s original description in that the proximal raphe ends are more ventral, the valves have narrower widths, and the valves have a narrower range in lengths.

The holotype illustration of Epithemia argus in Schmidt et al. (1904) has reflexed ends. On the same plate, however, Figs 2, 3, and 9 are named E. argus var. alpestris, but possess rounded ends.