Phylum: Charophyta   Family: Desmidiaceae

Cosmarium regnesi P.F. Reinsch 1866

A small desmid with semicells oblong-rectangular, isthmus elongate giving rise to wide open sinuses with a slightly rounded to almost square apex. Each semicell has 8 quite large granules, relative to the size of the cell, giving the apex and undulate appearance. It needs to be compared with the similar C. tritum (West) Kouwets 2025 and C. regnesiforme Van Westen & Coesel 2021.
Cell dimensions (3): L: 12.3-13.2 µm, B: 11.5-13.3 µm, Is: 4.4-5.6 µm; Th: 6.2-7.1 µm, L/B: 1.00-1.07.
Infrequently recorded but possibly overlooked. Found in acidic to mildly alkaline conditions such as loch-margins and abandoned peat-cuttings.

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.