Phylum: Charophyta   Family: Desmidiaceae

Cosmarium subtumidum  V.B. Nordstedt 1878

The semicells are described as trapezoid with convex sides. This was reasonably easy to identify but now we have C. pseudotumidum Coesel & Meesters 2023, whose semicells are described as more rectangular, with convex sides and rounded corners. The differences are subtle, and one can image many cells hard to distinguish. Both taxa have virtually the same cell dimensions. The latter taxon apparently is found in less acidic habitats.
Cell dimensions: L: 24.5-34 µm; B: 23-29 µm; Is: 8.2-11.5 µm; Th: 15-17 µm; L/B: 1.06-1.19.
Found in acidic, oligotrophic, habitats, such as moorland pools in bogs and abandoned peat-cuttings.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
Van Westen, M.C. (2024). Sieralgen in Drenthe.