Phylum: Charophyta   Family: Desmidiaceae

Haplotaenium rectum  (G.B. Delponte) T. Bando 1988

The main differences separating this genus from Pleurotaenium are the lack of terminal vacuoles, an axile chloroplast, a slight, but discernible, basal inflation and a smooth cell-wall. The similar H. minutum is generally smaller and doesn’t have a basal inflation, or is barely discernible, but still shows an obvious sinus.
Cell dimensions: L: 165-309 µm; B: 11-17.2 µm; L/B: 11-17.
Fairly common in peaty moorland pools and disused peat-cuttings.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.