Phylum: Charophyta   Family: Desmidiaceae

Staurodesmus subhexagonus  (W. & G.S. West) P.F.M. Coesel 1993

A small cell with an open sinus with a rounded or slightly notched extremity. Semicells subhexagonal to elliptic with broadly rounded lateral angles supporting small spines, which are parallel to slightly convergent. The apex is more or less truncate.
Cell dimensions: L: 17-22.5 µm; B: 17.8-24.8 µm; Is: 6.8-10.9 µm; Sp: 1.5-4.5 µm; Th: 9.2 µm; Overall B: 20-33.
A rare species only known in the Outer Hebrides from slightly acidic to more neutral waters such as shallow loch-margins and moorland pools.

References: 
Coesel, P.F.M. & Meesters, K.J. (2013). European Flora of the Desmid Genera Staurastrum and Staurodesmus.
Teiling, E. (1967). The Desmid Genus Staurodesmus. A taxonomic study.
West, W. & West, G.S. (1912). A Monograph of the British Desmidiaceae, Volume 4.