Phylum: Charophyta   Family: Desmidiaceae

Staurastrum duacense West & G.S. West 1909

Cells about twice as broad as long, including processes. Sinus apex acute, semicells vase-shaped broadening towards the apex. Apex board and straight with a pronounced row of emarginate verrucae. Processes tipped with two medium spines.
First described by West (1892, p. 184, pl. 24: 1) as Staurastrum pseudosebaldi subsp. duacense; later, West, West & Carter (1923, p. 116, pl. 148: 1) raised it to species level. Coesel & Meesters (2013, p. 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, p. 58, pl. 1: 20) as: ‘forma plus tumida’. Figure 3 is from Boldt (1885, p. 117, pl. 6: 36) as S. pseudosebaldi var. bicornis! West, West & Carter (l.c. p. 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 important? Is the number of spines at the ends of the processes significant? - one could argue yes as recent authors have used is as a differentiating character between varieties.

References: 
Boldt, R. (1885) Bidrag till kännedomen om Sibiriens Chlorophyllophycéer.
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.
Coesel, P.F.M. & Meesters, K.J. (2013) European Flora of the Desmid Genera Staurastrum and Staurodesmus.