Phylum: Charophyta   Family: Desmidiaceae

Cosmarium quadrum P. Lundell 1871

A large taxon that West & West (1912: 20) note "… the rarest of the conspersum-group in the British Isles, …"
Cell dimensions: L: 72-77 µm; B: 64-67 µm; Is: 22-31 µm; Th: 39 µm; L/B: 1.09-1.15.
Infrequently recorded in acidic to circumneutral waters: moorland pools, abandoned peat-cuttings and ditches.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
Van Westen, M.C. (2024). Sieralgen in Drenthe.
West, W. & West, G.S. (1912). A Monograph of the British Desmidiaceae, Volume 4.