Phylum: Charophyta   Family: Desmidiaceae

Staurastrum ralfsii (W. & G.S. West) P.F.M. Coesel & K.J. Meesters 2013

First described and illustrated by West & West (1912: 156, pl. 124: 12, 13, 15, 16) as Staurastrum orbiculare var. ralfsii. More recently Coesel & Meesters raised it to species rank. It is very similar to Staurastrum orbiculare being slightly smaller but much more common in the Outer Hebrides.
Cell dimensions: L: 35-47 µm; B: 30-37 µm; Is: 11.0-14.2 µm; L/B: 1.17-1.42.
Common in shallow, slightly acid water, such as moorland pools and abandoned peat-cuttings.

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