Neidium

Neidium are long symmetrical biraphid diatoms with tips of many different shapes. The central ends of the raphe, unlike many other biraphid diatoms, deflect to opposite sides if they deflect at all. Striae are composed of one-row areolae that are often pronounced. They often live in slightly acidic waters.

N. dubium

Cells are 30-48 µm in length, 14-16 µm in width.
Rows of striae are 16-24 per 10 µm.
Valves are almost rectangular, with short snub-nosed points.
Striae are evenly spaced and parallel at the center, becoming slightly radiate at the ends..
Raphe are straight and do not deflect to a side at the center.

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Raphe deflecting to opposite sides in the center of a valve

A3m1 Neidium dubium1_edited

A3m1 Neidium dubium 3_edited

A3m1 Neidium dubium_edited

Undetermined Neidium species

Specimens seem most close to N. sacoense or N. affine in terms of size range and striae density, but are too variable in morphology to say for sure.
Rows of striae range from 15.8-16.4 per 10 µm.