Phylum: Charophyta   Family: Desmidiaceae

Staurastrum pseudopelagicum W. & G.S. West 1903

First described and illustrated by West & West (1903: 547, pl. 18: 1-3). The authors note: “This species bears considerable resemblance to S. pelagicum, … but is distinguished by the different form of the semicells and the more open sinus and by the processes. The processes of S. pseudopelagicum are hollow, containing part of the cell-cavity, whereas those of S. pelagicum are solid projections of the cell-wall.” West, West & Carter (1923: 108) comment: “Some of the British specimens, however, have been observed … to possess processes which end in 3 spines, …” This phenomenon has been observed in the Outer Hebrides.
Overall cell dimensions: L: 40-55 µm; B: 50-80 µm; Is: 10-13.5 µm.
A rare planktonic species with a seemingly Atlantic distribution. Found in slightly acidic waters.

References: 
Coesel, P.F.M. & Meesters, K.J. (2013). European Flora of the Desmid Genera Staurastrum and Staurodesmus.
West, W. & West, G.S. (1903). Scottish freshwater plankton, No.I.
West, W., West, G.S. & Carter, N. (1923). A Monograph of the British Desmidiaceae, Volume 5.