Phylum: Charophyta   Family: Desmidiaceae

Staurastrum turgescens G. de Notaris 1867

A triradiate species with broadly rounded lateral angles making it relatively easy to identify.
Cell dimensions: L: 38.1-41.1 µm; B: 33.1-35 µm.; Is: 13.2-16.2 µm; L/B: 1.12-1.24.
A rare desmid. The current finds from the Outer Hebrides have come from semi-permanent rainwater pools in a disused quarry, which are slightly acidic to slightly alkaline.

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.