Compare

Biremis zachariasii has linear valves without any marginal undulation and more rounded ends in smaller valves.

LM scalebar = 10 µm
Biremis undulata LM1 Biremis undulata LM5 Biremis undulata LM4 Biremis undulata LM3 Biremis undulata LM2
Length Range
23.6-31.1 µm
Width Range
8.1-8.7 µm
Striae in 10 µm
11.9-12.5

Biremis circumtexta

(F.Meister ex Hust.) Lange-Bert. and Witkowski in Witkowski, Lange-Bert. and Metzeltin 2000

Biremis circumtexta LM3 Biremis circumtexta LM5 Biremis circumtexta LM2 Biremis circumtexta LM1 Biremis circumtexta LM7 Biremis circumtexta LM6 Biremis circumtexta LM4
Length Range
26-36 µm
Width Range
6.0-7.1 µm
Striae in 10 µm
16-18

Also compare to

Biremis zachariasii

(Reichelt) Edlund, N.Andresen and N. Soninkhishig