Phylum: Charophyta   Family: Desmidiaceae

Cosmarium difficile J. Lütkemüller 1893

A most variable species both in size and shape. A general characteristic is that the species displays parallel sides. However, these can be slightly to distinctly concave; conversely, they can be slightly rounded. It is worth bearing in mind that some cells might not have reached maturity and, also, environmental factors may inhibit or distort growth. Compare with two newly described species: C. perdifficile and C. pseudodifficile. I have tabulated several plates below to show this variability.
Cell dimensions: L: 23-38 µm, B: 16-23 µm, Is: 4.1-6.8 µm; Th: 11.4-16.6 µm, L/B: 1.35-1.67.
Widespread and common in acidic moorland pools and Sphagnum bogs.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium.