Neidium bisulcatum

Valves are linear and elongate, with rounded apices. A longitudinal canal is present along each valve margin. The valve face is flat. Areolae are elongate along the transapical axis.

Striae are parallel to slightly oblique across the valve face, becoming convergent near the apices. The angle of the striae differs at the apices, with the more strongly converging striae occurring on opposite sides of the raphe at each apex. The central area is oval to rhombic. Voigt discontinuities are subtle, when present.

Proximal raphe ends are long and reflexed in opposite directions toward the valve margins (~100°). Proximal raphe ends tend to extend to the edge of the central area. Specimens commonly display a single irregular raphe end, pointing anywhere from 0° to 180°.

The axial area expanded asymmetrically near apices. The raphe is an oblique slit.

Internally, valves bear a rectelevatum. Helictoglossae are robust, and are noticeably larger than those of Neidium capsulare. Lacinia are difficult to resolve.

The specimens documented in the Rocky Mountains of Colorado conform well to the original description of Lagerstedt (1873). This taxon, however, may have been confused with other similar taxa historically. In European literature, it has been included within a broad morphological species concept (Lange-Bertalot et al. 2017).

A number of Neidium taxa have similar linear, elongate valve margins. Particular attention should be paid to the valve outline, width, and proximal raphe ends.