Phylum: Charophyta   Family: Desmidiaceae

Cosmarium subprotumidum  C.F.O. Nordstedt 1876

First described and illustrated by Nordstedt (1876: 38, pl. 12: 14). Rather similar to a number of other cells, which has induced frequent misidentifications in the past.
Cell dimensions: L: 23-30 µm, B: 19.5-27 µm, Is: 6.7-10 µm; Th: 13.5-18 µm, L/B: 1.11-1.22.
Because of taxonomic confusion, its status is unclear. In the Outer Hebrides, it is infrequently recorded in circum-neutral waters.

References: 
Kouwets, F.A.C. (in manuscript). European Flora of the Desmid Genus Cosmarium.
Nordstedt, C.F.O. [in] Nordstedt, C.F.O. & Wittrock, V. (1876). Desmidieae et Oedogonieae ab O. Nordstedt in Italia et Tyrolia collectae, quas determinaverunt.