Neidium amphigomphus

Valves are large and linear to linear-elliptic, with cuneate apices. The valve face is flat. Valve margins are nearly parallel to slightly convex. Multiple longitudinal canals (3-5) are present along the valve margin. The axial area is straight to slightly expanded. In the largest specimens, the axial area may bear irregular markings on both sides of the raphe, while the axial area tends to be hyaline in small to medium sized specimens. The raphe is mostly oblique, becoming thread-like near the terminal end. Proximal raphe ends are small and strongly hooked in opposite directions. Distal raphe ends form lacinia, although this feature is difficult to resolve in LM. Internally, valves bear a rectelevatum. Helictoglossae are small, but can be resolved in focal deep planes.

The striae are slightly oblique across most of the valve face, but the angle of the striae differs bilaterally at the apices, with striae becoming parallel on opposite sides of the raphe at each apex. The central area is elliptic and slightly elongate along transapical axis. Voigt’s discontinuities, when present, are subtle. Areolae are relatively small and round near the axial area, becoming large and transapically elongate near the valve margin.

The smallest specimen presented here is shorter than the minimum length provided for this taxon other publications (Siver and Hamilton 2011, Lefebvre and Hamilton 2015). The maximum length observed in this population is roughly the same as the maximum length reported (Lefebvre and Hamilton 2015) in modern populations, but those authors also reported specimens up to 167 µm long and 42 µm wide in Ehrenberg's type material.

In SEM it can be resolved that areolae are open externally and internally are occluded by a hymen (Siver and Hamilton 2011). Renilimbia are located near areolae across the valve face (Lefebvre and Hamilton, 2015). The valve walls are chambered both apically and transapically (Lefebvre and Hamilton, 2015), similar to N. rudimentarum, N. ampliatum, N. gracile, N. hitchcockii, and N. tumescens (Hamilton et al. 2004).

Although the unique morphological features of this taxon have helped reduce the number of misidentifications compared to other large Neidium taxa such as N. iridis, some errors can be found in older literature. The specimens presented in Krammer and Lange-Bertalot (1986), for example, do not fit within the current morphological species concept (Lefebvre and Hamilton 2015).