Neidium iridis

Valves are large and lanceolate, with cuneate apices. A longitudinal canal is present on each valve margin. The valve face is relatively flat, however due to the large size, it may be difficult to get an entire specimen into focus.

Areolae are irregular outside of the longitudinal canal. Areolae are smaller near the axial area.

Striae are slightly oblique to the central sternum. Accordingly, the central area tends to be a slightly oblique oval. The angle of the striae differs bilaterally at the apices, with the more strongly converging striae occurring on opposite sides of the raphe at each apex. Voigt discontinuities are typically prominent.

Proximal raphe ends are small and variably hooked in opposite directions. In some specimens the proximal raphe ends are deflected at ~90°. In others, the proximal raphe ends curve ~180°, pointing nearly to the valve apices.

The axial area is constricted near the central area and apices. The axial area is only hyaline on one side of the raphe with the opposite side bearing a series of irregular areolae. Raphe is an oblique slit. Internally, valves bear a rectelevatum. Helictoglossae are robust enough to resolve using LM, and reflex towards the center of the valve.

In SEM, areolae are externally occluded by radiating cribra, and covered internally by hymenes (Hamilton et al. 2019). Distal raphe ends are difficult to resolve using LM, but under SEM, it can be distinguished that this taxon bears lacinia.

This taxon is large in size, comparable to the large species of Pinnularia (Hamilton et al. 2019). Even at low magnification, specimens can be readily identified due to the size and golden-orange iridescence under illumination. Specimens as large as 320 µm have been identified (Hamilton et al. 2019). However, other large Neidium have been confused with N. iridis.

In most assessments in North America, a rather broad species concept has been applied to this taxon. Specimens previously identified as N. iridis should be reevaluated with our modern concept of this species. Those that do not fit this description should be treated as separate taxa, as they are likely new or undescribed species.