Phylum: Charophyta   Family: Desmidiaceae

Micrasterias denticulata  J. Ralfs 1848

It should be carefully compared with M. thomasiana var. notata, which is similar in size and ecology. That species has three protuberances near the base of the semicells. However, these are often vestigial and hardly noticeable. One small difference is the apical lobe, smooth in M. denticulata, sporting a small spicule in M. thomasiana var. notata.
Cell dimensions: L: 230-280 µm; B: 191-237 µm; Is: 31-40 µm; L/B: 1.18-1.23.
West & West (1905) record it as one of the commonest British species, including pure collections from boggy ditches and springs. It is still fairly common in mesotrophic waters such as peaty pools with shallow vegetation, and Sphagnum 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.