Compare

Neidium capsulare is more narrow (typically <6.5 ┬Ám) and has shorter proximal raphe ends.

Neidium stoermeri has triundulate margins and a broader, more rectangular central area.

LM scalebar = 10 µm
Neidium bisulcatum LM5 Neidium bisulcatum LM1 Neidium bisulcatum LM2 Neidium bisulcatum LM3 Neidium bisulcatum LM7 Neidium bisulcatum LM4 Neidium bisulcatum LM6
Length Range
37.2-71.7 µm
Width Range
6.5-8.7 µm
Striae in 10 µm
29-33

Also compare to

Neidium stoermeri

Thomas and Kociolek