Phylum: Charophyta   Family: Desmidiaceae

Cosmarium caelatum J. Ralfs 1848

A distinct species and difficult to confuse with any other. In face-view the semicells have an exaggerated crenate outline. The basal lobe supports 4-5 quite large, pointed, granules. The upper lateral lobe has 3-4 slightly smaller granules. The truncate apex supports 4 raised lobes with up to three granules each. It has a broad central tumour, which is best seen in apical view.
Cell dimensions: L. 43-46 µm; B. 37-40 µm; Is. 15-18 µm; Th. 23-27 µm; L/B. 1.11-1.21.
A widespread species most readily found in upland boggy areas and with moss on wet rocks but never abundant.

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