Phylum: Charophyta   Family: Desmidiaceae

Cosmarium biretum L.A. de Brébisson ex J. Ralfs 1848

First described by Brébisson in Ralfs (1848: 102, pl. 16: 5) and illustrated by Jenner.
Cell dimensions: L: 38-84 µm; B: 35-69 µm; Is: 13-27 µm; Th: 23-48 µm; L/B: 0.95-1.28.
Regarded as fairly common in slightly acidic waters such as moorland pools and loch-margins, but proving to be scarce in the Outer Hebrides.

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.
Ralfs, J. (1848). The British Desmidieae.