Phylum: Charophyta   Family: Desmidiaceae

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

One of many undulate species but little to confuse it with. Semicells are pyramidate with about 18 undulations in face view, these get progressively larger towards the apex. Perhaps the closest in resemblance is C. benedictum Ducellier 1919 but that species has less pronounced undulations, a slightly open sinus and a larger L/B ratio.
Single cell dimensions: L: 39.1 µm; B: 26.2 µm; Is: 15.1 µm; Th: c.19-20 µm; L/B: 1.50.
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 Westen, M.C. (2024). Sieralgen in Drenthe.