Phylum: Charophyta   Family: Desmidiaceae

Micrasterias truncata var. crenata (J. Ralfs) R. Grönblad 1920

Smaller than the nominate with rounded polar and upper lateral lobes, although intermediates exist.
Single cell dimensions: L: 91 µm; B: 86 µm; Is: 17.2 µm; L/B: 1.06.
Found in acidic mesotrophic waters such as moorland pools and abandoned peat-cuttings. Rarely encountered.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
West, W. & West, G.S. (1905). A Monograph of the British Desmidiaceae, Volume 2.