Phylum: Charophyta   Family: Desmidiaceae

Cosmarium cataractarum (M. Raciborski) B. Eichler 1895

A medium-sized cell with pyramidal-shaped semicells with straight to slightly convex sides and a small, often dented, apex. In, or near, the centre is a scrobiculus, but this is not always easy to see with a light microscope and, apparently, can be lacking. Compare with the recently described C. reticulatum Lilitskaya 2012. See Kouwets (2025: 558) for more details.
Cell dimensions (2): L: 41.5-42.2 µm; B: 29.3 µm; Is: 10 µm; Th: c.18; L/B: 1.42-1.44.
It seems to prefer neutral to alkaline, meso-eutrophic habitats but, so far, rarely encountered in the Outer Hebrides.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium.
Van Westen, M.C. (2024). Sieralgen in Drenthe.