Phylum: Charophyta   Family: Mesotaeniaceae

Mesotaenium macrococcum (F.T. Kützing) J. Roy & J.P. Bisset 1894

Similar to M. chlamydosporum in size and habitat but the chloroplast in that species fills the cell in face-view. In M. macrococcum the chloroplast has a toothed margin and doesn’t quite reach the cell-wall.
Cell dimensions: L: 15-37 µm; B: 8-17 µm; L/B: 1.4-3.1.
Found in atmophytic habitats such as dripping rocks and on moss and other substrates, but also in abandoned peat-cuttings and moorland bogs and pools.

References: 
Brook, A.J. & Williamson, D.B. (2010). A Monograph on some British Desmids.
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
Van Westen, M.C. (2024). Sieralgen in Drenthe.