Phylum: Charophyta   Family: Desmidiaceae

Staurodesmus subtriangularis  (O. Borge) E. Teiling 1948

Semicells are roughly triangular with a slight indentation on the lateral sides. The apex is raised usually with a shallow central depression. The sinus is wide open and can sometimes appear slightly elongated. The longish spines are either parallel or slightly divergent.
Cell dimensions: L: 25-37.6 µm; B: 24-27 µm; Is: 7.5-10.2 µm; Sp: 6-22 µm; Overall LxB: 26-42 x 45-65.
A rare species in Europe, only known from Britain. It would suggest an Atlantic distribution. In the Outer Hebrides it has been found in both shallow and deeper lochs with varying degrees of acidity.

References: 
Coesel, P.F.M. & Meesters, K.J. (2013). European Flora of the Desmid Genera Staurastrum and Staurodesmus.