Phylum: Charophyta   Family: Desmidiaceae

Staurodesmus crassus (W. & G.S. West) M.-B. Florin 1957

A small cell with wide-open sinuses from an angular or rounded base. Semicells sub-cuneate with a convex, or occasionally, truncate apex. Tiny spines occur at the apical angles pointing upwards. Cells are biradiate. Coesel & Meesters (2013: 20) made this a variety of the smaller Std. controversus even though they note there is no overlap in cell dimensions. West & West (1912: 102), as Arthrodesmus crassus, also note a difference in the apical view: broadly elliptic-fusiform in var. crassus, broadly elliptic in the latter. I therefore prefer to follow Teiling (1967: 503) and keep them as separate taxa.
Cell dimensions (2): L: 15-19.4 µm; B: 15.5-16 µm.; Is: 8.5-9 µm; Sp: 1-3.4; Overall B: 18-20.7.
A rare species, most likely to be found in Atlantic regions in oligotrophic acidic waters.

References: 
Coesel, P.F.M. & Meesters, K.J. (2013). European Flora of the Desmid Genera Staurastrum and Staurodesmus.
Teiling, E. (1967). The desmid genus Staurodesmus. A taxonomic study.
West, W. & West, G.S. (1912). A Monograph of the British Desmidiaceae, Volume 4.