Phylum: Charophyta   Family: Desmidiaceae

Cosmarium galeatum A. van Geest & P.F.M. Coesel 2019

One of many crenate species, which can be confusingly similar. Semicells are broadly pyramidal with the slightly convex sides merging with the apex. Each semicells has 18 crenations when seen in face view. Cells are almost twice as long as broad giving a quite large L/B ratio. Compare with the similar C. fruticosum Van Geest & Coesel 2019.
Single cell dimensions: L: 46.1 µm, B: 25.7 µm, Is: 13.9 µm; Th: c.18-19 µm, L/B: 1.80.
Status in the Outer Hebrides is unknown but seemingly scarce. It would seem to occur predominantly in atmophytic habitats such as ephemeral pools and with moss on wet rocks.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium. Part 1: Text. Part 2: Plates.
Van Geest, A. & Coesel, P.F.M. (2019). Some new and interesting desmids (Streptophyta, Desmidiales) from ephemeral puddles in the urban and industrial areas of Amsterdam (Netherlands).