Phylum: Charophyta   Family: Desmidiacea

Cosmarium sphalerostichum C.F.O. Nordstedt 1876

Semicells are trapeziform with a closed sinus; the sides are convex displaying 5 granules including the apical angle (with a second series when seen in face view … the granules vary in size between populations). The truncate apex has 3 submarginal granules, excluding the apical angle, giving an undulate effect. It needs to be compared with the morphologically similar C. furcatospermum, however, that species occupies a different environment and has more rectangular semicells also, the L/B ratio is smaller.
Cell dimensions: L: 17-20 µm; B: 15-17 µm; Is: 6-7 µm; Th: 10-12 µm; L/B: 1.16-1.26.
A rarely recorded Arctic-alpine species probably confined to the north-west in Britain, although West & West (1908: 176) has it more widespread in mountainous regions. The habitat is atmophytic with the Outer Hebridean records coming from moss on dripping rocks.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium.
Van Westen, M.C. (2024). Sieralgen in Drenthe.
West, W. & West, G.S. (1908). A Monograph of the British Desmidiaceae, Volume 3.