Phylum: Charophyta   Family: Desmidiaceae

Cosmarium quadratum J. Ralfs 1848

A widespread and common species complex. Kouwets (2025) has now split it into seven separate species based on size, morphology and ecology.
Kouwets' cell dimensions: L: 50-71 µm; B: 29-37 µm; Is: 20-26 µm; Th: 25-30 µm; L/B: 1.7-1.9.
Because of the foregoing the status is now uncertain. It prefers slightly acidic to slightly alkaline habitats such as moorland pools and loch-margins. It has also been found in ephemeral winter pools.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium. Part 1: Text. Part 2: Plates.