Phylum: Charophyta   Family: Desmidiaceae

Cosmarium cedercreutzii R.L. Grönblad 1921

There are several crenate species to cause possible confusion, and it needs to be carefully compared with the recently described C. neonotabile Van Westen & Coesel 2021 and C. pseudosaxicola Kouwets 2025. That latter species is slightly smaller with a lower L/B ratio. It also supports some flattened warts in the centre of the semicells, but these can be difficult to see and can be reduced.
Cell dimensions: L: 25.5-29.8 µm; B: 17.5-21.2 µm; Is: 10-13.2 µm; Th: 14.5-16; L/B: 1.34-1.51.
It occurs with moss on wet rocks and similar oligotrophic habitats. Distribution is little known due to earlier taxonomic confusion.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium.