Phylum: Charophyta   Family: Desmidiaceae

Cosmarium pyramidale (P.F.M. Coesel) P.F.M. Coesel & K.J. Meesters 2023

Rather similar to a number of other cells, which has caused frequent misidentifications in the past. Compare with C. subprotumidum Nordstedt 1876, which is of similar size and profile with an elevated apex. However, the apex on that species is greater than half the size of the breadth of the cell; no more than half in C. pyramidale.
Single cell dimensions: L: 23.7 µm, B: 20.8 µm, Is: 7.5 µm; Th: c.13.2 µm, L/B: 1.14.
Because of taxonomic confusion, its status is unclear. In the Outer Hebrides, it is rarely recorded.

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