Phylum: Charophyta   Family: Desmidiaceae

Cosmarium subgranatum var. borgei Willi Krieger 1944

This variety differs from the nominate in having hemispherical semicells rather than pyramidal. Most cells found in the Outer Hebrides have 10 undulations per semicell but occasionally cells with 8 undulations are encountered; these can be confused with C. impressulum.
Cell dimensions: L. 19-25 µm; B. 14-22 µm; Is. 4.5-8.2 µm; Th. 13-14 µm; L/B. 1.2-1.5.
Fairly common in the plankton of circumneutral waters.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
Kouwets, F.A.C. (in manuscript). European Flora of the Desmid Genus Cosmarium.
Krieger, W. (1944). Süßwasseralgen aus Griechenland.
Van Westen, M. (2024). Sieralgen in Drenthe.