Phylum: Charophyta   Family: Desmidiacea

Staurodesmus glaber var. limnophilus E. Teiling 1967

It differs from the nominate variety in having a convex apex and longer convergent spines, but intermediates exist that are hard to place. In apical view the sides are virtually straight.
Cell dimensions: L: 18-23 µm; B: 17-22 µm; Is: 6-8 µm; Sp: 8-10 µm; Overall B: 29-37.
Seemingly scarce in the Outer Hebrides but may have been confused with the nominate variety in the past. Found in acidic moorland bogs, abandoned peat-cuttings, and vegetated loch-margins.

References: 
Coesel, P.F.M. & Meesters, K.J. (2013). European Flora of the Desmid Genera Staurastrum and Staurodesmus.
Van Westen, M.C. (2024). Sieralgen in Drenthe.