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 lines(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 the 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.

Ehrenberg's type material contained specimens up to 167 µm long and 42 µm wide (Lefebvre and Hamilton 2015). The smallest specimen presented here is smaller than the minimum length reported elsewhere (Siver and Hamilton 2011, Lefebvre and Hamilton 2015). The maximum length observed in this population close to the maximum length reported in modern populations (Lefebvre and Hamilton 2015).

Externally, areolae are open. Internally, areolae 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 clarification of morphological features has helped reduce errors in identification of large Neidium, note that older literature may be misleading. For example, specimens presented in some sources (Krammer and Lange-Bertalot 1986), differ from more recent species boundaries (Lefebvre and Hamilton 2015).