Phylum: Charophyta   Family: Desmidiaceae

Staurastrum furcigerum (J. Ralfs) W. Archer 1861

A common but variable taxon. West, West & Carter (1923) recognised several varieties, usually based on the variability of the processes. As intra-specific cells have been found, it is generally accepted that this is variability within one species. West & West (1906: 104) recorded a smaller variety, S. furcigerum var. reductum, which might be an exception. It has an exceptionally broad isthmus at 21 µm. This was recognised from finds in Ireland but nothing similar has been found in the Outer Hebrides to date.
Overall cell dimensions: L: 57-71 µm; B: 53-65 µm; Is: 11.2-17.6 µm; L/B: 1.04-1.11.
Favours circumneutral waters such as loch-margins, 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, G.S. & West, W. (1906). A comparative study of the plankton of some Irish Loughs.
West, W., West, G.S. & Carter, N. (1923). A Monograph of the British Desmidiaceae, Volume 5.