Phylum: Charophyta   Family: Desmidiaceae

Cosmarium exiguum W. Archer 1864

First described and illustrated by Archer (1864: 178, pl. 6: 32-33). West & West (1908: 63) specifically mention this species as being “Common in the Outer Hebrides”. Recent sampling would suggest it's still reasonably common; not surprising as there is ample suitable habitat.
Cell dimensions: L: 22-27.4 µm, B: 11-13 µm, Is: 4-5.5(-7.1) µm; Th: 8.5-11.8 µm, L/B: 1.7-2.3.
Found in acidic bogs, especially with Sphagnum.

References: 
Archer, W. (1864). Description of two new species of Cosmarium (Corda) of Penium (Bréb.) and of Arthrodesmus (Ehr.).
Kouwets, F.A.C. (in manuscript) European Flora of the Desmid Genus Cosmarium.
West, W. & West, G.S (1908). A Monograph of the British Desmidiaceae, Volume 3.