Phylum: Charophyta   Family: Desmidiaceae

Staurastrum duacense W. & G.S. West 1909

First described by West (1892: 184, pl. 24: 1) as Staurastrum pseudosebaldi subsp. duacense; later, West, West & Carter (1923: 116, pl. 148: 1) raised it to species level. Coesel & Meesters (2013: 92, pl. 101: 1-3) note: “… is an incidentally recorded, most questionable species. Probably it is a mere biradiated form of S. manfeldtii …” Their first figure is a reproduction from West, West & Cater. Figure 2 is from Gronvold (1945: 58, pl. 1: 20) as: “forma plus tumida”. Figure 3 is from Boldt (1885: 117, pl. 6: 36) as S. pseudosebaldi var. bicornis! West, West & Carter (l.c.: 117) note “It is very similar in appearance to S. pseudosebaldi Wille var. bicornis but differs in the more complicated lateral margins of the semicells.”
I would add that the true S. duacense (as described and illustrated by West, West & Carter) has the processes tipped with only two medium size spines. The other taxa mentioned here have 3-4 small spines at the ends of the processes.
More finds are needed to get a better understanding of these biradiated cells. Are they taxonomically importantµm Is the number of spines at the ends of the processes significantµm - one could argue yes as recent authors have used it as a differentiating character between varieties.
Cell dimensions: L. 32-42 µm, B. 60-85 µm, Is. 9-11 µm.
A seemingly rare taxon but the picture is confounded by taxonomic confusion.

References: 
Boldt, R. (1885). Bidrag till kännedomen om Sibiriens Chlorophyllophycéer.
Coesel, P.F.M. & Meesters, K.J. (2013). European Flora of the Desmid Genera Staurastrum and Staurodesmus.
Grönblad, R. (1945). A further report of freshwater-algae recorded from the neighbourhood of the zoological station at Tvärminne.
West, W., West, G.S. & Carter, N. (1923). A Monograph of the British Desmidiaceae, Volume 5.