Phylum: Charophyta   Family: Desmidiaceae

Cosmarium blyttii var. novae-sylvae W. & G.S. West 1897

It differs from the nominate variety in its more elaborate central granulation. However, there is considerable variation between and within the varieties, which makes it difficult to separate then, suggesting a single variable species. The nominate variety is said to be slightly smaller but with considerable overlap with the dimensions, this may not be significant either. As part of this ornamental variation the supraisthmial granules can be single or double.
Cell dimensions: L: 18-23 µm, B: 14-18 µm, Is: 4.5-6 µm; Th: 9-10 µm; L/B: 1.2-1.3.
Widespread in Britain in vegetated margins of lochs and bogs and well represented in the Outer Hebrides.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium.
West, W. & West, G.S. (1908). A Monograph of the British Desmidiaceae, Volume 3.