Phylum: Charophyta   Family: Desmidiaceae

Cosmarium subprotumidum  C.F.O. Nordstedt 1876

Rather similar to a number of other cells, which has caused frequent misidentifications in the past. Compare with C. pyramidale (Coesel) Coesel & Meesters 2023, which is of similar size and profile with an elevated apex. However, the apex on that species is no more than half the size of the breadth of the cell; greater in C. subprotumidum.
Single cell dimensions: L: 24.8 µm, B: 20.5 µm, Is: 7.2 µm; Th: c.15 µm, L/B: 1.21.
Because of taxonomic confusion, its status is unclear. In the Outer Hebrides, it is rarely recorded.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium. Part 1: Text. Part 2: Plates.