Phylum: Charophyta   Family: Desmidiaceae

Cosmarium orthostichum  P. Lundell 1871

Quite a distinct desmid with sub-semicircular semicells with about 16 granules visible in face-view. The cell-wall is ornamented with vertical rows of granules, many divided. The few records from the Outer Hebrides present dimensions a little smaller than those presented by Kouwets (2025). They align more closely with those presented by John & Williamson (2009) from western Ireland.
Cell dimensions: L: 31.4-32.2 µm; B: 26.5-28.5 µm; Is: 10.7-11.3; Th: c.21 µm; L/B: 1.11-1.21.
A widespread but rare species largely known from north-western Europe. Recent finds in the Outer Hebrides are from slightly acidic waters.

References: 
John, D.M. & Williamson, D.B. (2009). A Practical Guide to the Desmids of the West of Ireland.
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium. Part 1: Text. Part 2: Plates.
West, W. & West, G.S. (1908). A Monograph of the British Desmidiaceae, Volume 3.