Phylum: Charophyta   Family: Desmidiaceae

Cosmarium hexalobum f. longum E.K. Kosinskaya 1933

Kosinskaya (1933) from Russia was the first to describe these longer and narrower cells of the nominate as forma longum. There have been a number since which have been provisionally corralled under this forma. The sides are five crenate and the apex, including the enlarged apical angle, is four crenate.
Cell dimensions (2): L: 41.3-42.7 µm; B: 28-28.4 µm; Is: 16.2-16.8 µm; Th: 19.7-20.6 µm; L/B: 1.48-1.51.
A rare taxon with just one population found on the Outer Hebrides, from a granite depression subject to desiccation. The European distribution suggests an Arctic-alpine preference.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium. Part 1: Text. Part 2: Plates.