Phylum: Charophyta   Family: Desmidiaceae

Staurodesmus extensus  (O.F. Andersson ex O. Borge) E. Teiling 1948

First described and illustrated by Anderson in Borge 1890: 13, fig. 7) as Arthrodesmus incus var. extensus. It was later moved to its current genus by Teiling (1948: 67) and raised to species level. The spines are divergent in this nominate variety.
Cell dimensions: L: 19-23 µm; B: 15-18 µm; Is: 6-7.5 µm; Sp: 9-17 µm; Overall LxB: 29-40 x 31-49.
Common in mildly acidic to neutral waters, in the plankton and marginal vegetation of lochs.

References: 
Anderson, O.F. [in] Borge, O. (1890). Bidrag till kännedomen om Sveriges Chlorophyllophyceer. I. Chlorophyllophyceer från Roslagen.
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
Teiling, E. (1948). Staurodesmus, genus novum. Containing monospinous Desmids.