Eunotia luna var. aequalis Hustedt sensu Frenguelli. Original 1933

Frenguelli (1933) provided a brief description of E. luna var. aequalis, pointing that specimens from Iberá showed a lower stria density than those originally described by Hustedt (Table 1). For describing E. luna var. aequalis f. paucistriata, Frenguelli only mentioned that it differed from E. luna var. aequalis by its more distantly and irregularly spaced striae.

The valve of E. luna var. aequalis illustrated by Frenguelli (here reproduced in Fig. 1) has a convex dorsal margin and a slightly concave to almost straight ventral margin, ends broadly cuneate and terminal nodules moderately distant from the poles. The striae are evenly spaced, alternate with very short striae at both dorsal and ventral margins, and become radiate and more densely arranged towards the poles. In comparison, the valve of E. luna var. aequalis f. paucistriata depicted by Frenguelli (here reproduced in Fig. 2) is slightly more arcuate, with ends nose-like protracted and barely ventrally deflected. The striae are less dense and irregularly distributed, alternate with distinct short striae at both margins, and become abruptly denser at the apices.

After examining with LM the slides corresponding to the series above mentioned, we found valves ranging from large and more arcuate to small and with the ventral margin almost straight (Figs 3–15). Regarding the striation pattern, some of these specimens can be affiliated either to E. luna var. aequalis (Figs 4, 8–9, 11) or to E. luna f. paucistriata (Figs 5, 13, 15), but other ones exhibit intermediate morphologies and could not be undoubtedly assigned to one of them (Figs 3, 6–7, 10, 12, 14), thus hindering a clear differentiation of both taxa (see Table 1 for morphometric data). Examined with SEM (Figs 16–26), external valve face is smooth with uniseriate striae, parallel to slightly radial (Figs 16–18). At the apices striae become strongly radial and surround the terminal raphe fissure (Fig. 19). Dorsal valve mantle is perpendicular to valve face and shows striae more densely arranged (Fig. 20), some of which extend a short way onto the valve face. Valve face is delimited from ventral mantle by a marginal, unornamented ridge (Figs 16–18). Areolae are round, small and located in shallow depressions with scalloped margins (Fig. 19). Raphe sternum narrow with a filiform raphe positioned on the ventral mantle (Fig. 26), with distal endings turned towards the valve face (Fig. 19). The areolation pattern in the mantle between the raphe and the valve margin is denser and more disorganized than in the rest of the mantle (Fig. 26, arrows). No spines were observed. Internally areola openings are round and located in narrow troughs (Figs 21–25). Raphe terminates in well-developed helictoglossae, positioned ventrally on the valve face and somewhat distant from the apices (Fig. 24). One rimoportula apically positioned on the valve mantle (Fig. 21), opening internally through two parallel, sessile lips (Fig. 24); its external opening is indistinct. None of the specimens observed with SEM could be undoubtedly ascribed to either of the two taxa reported by Frenguelli.