Phylum: Charophyta   Family: Desmidiaceae

Cosmarium tinctum var. subretusum E. Messikommer 1942

First described and illustrated by Messikommer (1942: 145, pl. 5: 5). It differs from the nominate variety in have a retuse apex and a slight swelling in apical view, rather than a protuberance.
Cell dimensions: L: 8-13.5 µm; B: 7.5-12 µm; Is: 5-7 µm; Th: 5-7 µm; L/B: 0.9-1.25.
Found in similar habitats to the nominate variety but also in less acidic conditions. It would appear to be scarcer.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
Kouwets, F.A.C. (in manuscript). European Flora of the Desmid Genus Cosmarium.
Messikommer, E. (1942). Beitrag zur Kenntnis der Algenflora und Algenvegetation des Hochgebirges um Davos.