Phylum: Charophyta   Family: Desmidiaceae

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

First described and illustrated by W. & G.S. 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 predominantly biradiated but can also occur as triradiate. In apical view, it has a prominent protuberance, which is dentate.
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.