Phylum: Charophyta   Family: Desmidiaceae

Cosmarium tinctum  J. Ralfs 1848

First described by Ralfs (1848: 95, pl. 32: 7) and illustrated by Jenner. A variable species which has a number of varieties. The central protuberance as seen in apical view is also variable.
Cell dimensions: L: 8-13.5 µm; B: 7.5-12 µm; Is: 5-7 µm; Th: 5-7 µm; L/B: 0.9-1.25.
An acidophile of moorland pools and Sphagnum bogs, frequently recorded 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.
West, W. & West, G.S. (1905). A Monograph of the British Desmidiaceae, Volume 2.