Phylum: Charophyta   Family: Desmidiaceae

Cosmarium formosulum  R.T. Hoff 1888

A variable species both in terms of its size and central protuberance. It’s considered a species-complex and has given rise to new species being recently described. It should be carefully compared with C. pseudoformosulum Van Westen 2015, which has a more disparate arrangement of central granules.
Cell dimensions: L: 38-44.6 µm; B: 32.6-41.9 µm; Is: 11-14.6 µm; Th: 21.1-24.2 µm; L/B: 1.07-1.24.
Widespread and rather common in slightly acidic to mildly alkaline waters.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium. Part 1: Text. Part 2: Plates.
Van Westen, M.C. (2024). Sieralgen in Drenthe.