Phylum: Charophyta   Family: Desmidiaceae

Staurastrum arcuatum C.F.O. Nordstedt 1873

First described and illustrated by Nordstedt (1873: 36, pl. 1: 18). Coesel & Meesters (2013) describe the semicells as bowl-shaped with relatively long processes. However, the cell-shape is quite variable, especially the basal part of the semicell, which can have some angularity.
Cell dimensions: L: 34-40 µm; B: 32-41 µm; Is: 8.5-10.5 µm.
A tychoplanktonic species described as common on the continent. It is proving to be scarce in the Outer Hebrides.

References: 
Coesel, P.F.M. & Meesters, K.J. (2013) European Flora of the Desmid Genera Staurastrum and Staurodesmus.
Nordstedt, C.F.O. (1873) Bidrag till kännedomen om sydligare Norges Desmidiéer.
West, W., West, G.S. & Carter, N. (1923) A Monograph of the British Desmidiaceae, Volume 5.