Phylum: Charophyta   Family: Desmidiaceae

Cosmocladium saxonicum A. De Bary 1865

Cells longer than broad, sinus fairly deep and opening widely. Semicells in side-view orbicular, chloroplast axile with a single pyrenoid. Colonies are free-floating and can have a large number of cells. The largest of the British species.
Cell dimensions: L: 21-27 µm; B: 17-20 µm; Is: 5-7.5 µm; Th: 11-14 µm.
West, West & Carter (1923: 202, pl. 158: 13-16) specifically mention Harris in the Outer Hebrides as one of the sites where the species has been found. A rare species, found in nutrient-poor waters such as acidic loch-margins.

References: 
Bary, A. de (1865) Über Cosmocladium.
Coesel, P.F.M. & Meesters, K.J. (2023 second edition) Desmids of the Lowlands.
Prescott, W.G. et al. (2001 reprint) A Synopsis of North American Desmids. Part II, Desmidiaceae: Placodermae, Section 3.
West, W., West, G.S. & Carter, N. (1923) A Monograph of the British Desmidiaceae, Volume 5.