Phylum: Charophyta   Family: Desmidiaceae

Staurodesmus extensus  (O. Borge) E. Teiling 1948

Predominantly a biradiated species with an extended isthmus giving wide-open sinuses with a broadly rounded apex. Semicells are subrectangular above the isthmus with long divergent spines from the apical angles.
Cell dimensions: L: 16-22 µm; B: 15-17 µm; Is: 5.8-7.2 µm; Sp: 8-17 µm; Overall LxB: 30-40 x 30-48.
Quite common in mildly acidic waters, in the plankton and marginal vegetation of lochs. It has also been taken from more acidic, boggy terrain.

References: 
Coesel, P.F.M. & Meesters, K.J. (2013). European Flora of the Desmid Genera Staurastrum and Staurodesmus.
Van Westen, M.C. (2024). Sieralgen in Drenthe.