Phylum: Charophyta   Family: Desmidiaceae

Staurastrum simonyi var. semicirculare P.F.M. Coesel 1996

First described and illustrated by Coesel (1996: 25, figs 27-28). The varietal name describes the cell-shape precisely, it also has less ornamentation than the nominate variety.
Overall cell dimensions: L: 18-28 µm; B: 20-30 µm; Is: 5.5-7.5 µm.
Recorded as widespread for Europe but proving scarce in the Outer Hebrides.

References: 
Coesel, P.F.M. (1996). Taxonomic notes on Dutch desmids III.
Coesel, P.F.M. & Meesters, K.J. (2013). European Flora of the Desmid Genera Staurastrum and Staurodesmus.