Cymbella neocistula var. islandica

Valves are dorsiventral, with broadly rounded apices. The dorsal margin is strongly arched. In larger valves, the ventral margin is concave with a distinctly gibbous center. In smaller valves, the ventral margin is somewhat convex to nearly flat. The axial area located just ventral to the valve mid line. The central area is small and elliptic. The raphe is lateral, becoming reverse-lateral near the proximal ends. Proximal raphe ends are deflected ventrally and terminate in slightly expanded pores. Three to seven stigmata are present on the ventral side of the central area. Striae are radiate throughout. Areolae are distinct in LM and number 16-21 in 10 µm.

Patrick and Reimer (1975) report similar specimens from Lake Michigan (pl. 11, figs 5-7) as Cymbella cistula var. gibbosa. That taxon was described by Brun (1895) from Lake Geneva, Switzerland and was later elevated to species rank (Meister 1912) as Cymbella gibbosa. That name, however, is a later homonym of a species described by Pantocsek (1901). Krammer (2002, p. 79) proposed that the Brun taxon represents initial cells of C. cymbiformis and is not related to C. cistula. We therefore conclude that the specimens shown here from Montana, and those reported by Patrick and Reimer from Lake Michigan probably belong to C. neocistula var. islandica and not to the Brun taxon.

Reimer comments that stigmata may also be present on the dorsal side of the central area, and that valves may be up to 26 µm wide (Patrick and Reimer 1975, p. 63).