Phylum: Charophyta Family: Desmidiaceae
Staurastrum simonyi var. semicirculare Coesel
The varietal names describes the cell-shape precisely, it also has less ornamentation than the nominate variety. Said to be widespread but no data available for the Outer Hebrides
References:
Coesel, P.F.M. & Meesters, K.J. (2013) European Flora of the Desmid Genera Staurastrum and Staurodesmus