Phylum: Charophyta   Family: Desmidiaceae

Tetmemorus laevis var. minutus (A. de Bary) W. Krieger 1937

The only distinction between this variety and the nominate is its smaller size. However, there is overlap in cell dimensions making it impossible to categorise some cells. I have found both taxa in the same sample perhaps supporting the hypothesis that a single taxon with a broad size-range is feasible.
Cell dimensions: L: 45-70 µm; B: 17-21 µm; L/B: 2.8-3.7.
It is less common than the nominate variety in the Outer Hebrides (but bear in mind the comment on size overlap above). Found in acidic habitats, especially with Sphagnum.

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