Phylum: Charophyta   Family: Desmidiaceae

Staurastrum tetracerum var. irregulare (W. & G.S. West) A.J. Brook 1982

First described and illustrated by West & West (1894: 12, pl. 2: figs. 49-50) as St. irregulare. Much later, Brook (1982: 263, fig. 2) made it a variety of St. tetracerum. Brook (l.c.) described a collection from Wales: "… differing from S. tetracerum in that its processes tend to be longer, and especially in that the cell body possesses a wide, open sinus. ... In many specimens there are prominent granules, or occasionally a spine, at each corner of the semicell body where it bends sharply above the isthmus to join the base of each process." Cells are biradiated; in apical view, it has a prominent protuberance, which is dentate.
Overall cell dimensions: L: 23-40 µm; B: 20-46 µm.; Is: 4.9-7.7 µm; Th: 9-10 µm.
Fairly common in the circumneutral waters such plankton of lochs and moorland pools.

References: 
Brook, A.J. (1982). Desmids of the Staurastrum tetracerum-group from a eutrophic lake in mid-Wales.
Coesel, P.F.M. & Meesters, K.J. (2013). European Flora of the Desmid Genera Staurastrum and Staurodesmus.
West, W. & West, G.S. (1894). New British freshwater algae.