Phylum: Charophyta   Family: Desmidiaceae

Micrasterias denticulata var.angulosa (P.F. Hantzsch) W. & G.S. West 1902

Differs from the nominate variety in having the basal angles narrower than the median part resulting in a slightly higher L/B ratio. I’m not sure these minor differences, which are variable, justify varietal status.
Cell dimensions (2): L: 255-273 µm; B: 203-212 µm; Is: 42 µm; L/B: 1.26-1.29.
Infrequently found in mesotrophic waters such as vegetated loch-margins and moorland bogs.

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.