Phylum: Charophyta   Family: Desmidiaceae

Staurodesmus extensus  (O. Borge) E. Teiling 1948

Predominantly a biradiated species with the spines divergent in this nominate variety.
Cell dimensions: L: 16-23 µm; B: 13-18 µm; Is: 5.3-7.5 µm; Sp: 6-17 µm; Overall LxB: 22-40 x 24-49.
Common in mildly acidic to neutral waters, in the plankton and marginal vegetation of lochs.

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.