Phylum: Charophyta   Family: Desmidiaceae

Teilingia excavata  (J. Ralfs) P. Bourrelly 1964

First described and illustrated by Ralfs (1848: 67, pl. 6: 2) as Sphaerozosma excavatum. It was later moved by Bourrelly (1964: 190, fig. 10) to Teilingia. Cells about as long and broad. Apart from four apical granules (which are often difficult to see), the cell-wall is smooth.
Cell dimensions: L: 7-12 µm; B: 6-10 µm; Is: 4-6.5 µm; Th: 5-7 µm; L/B: 0.90-1.15.
West, West & Carter (1923: 211, pl. 160; 1-3) note for Scotland that it is frequently found and often conjugating. It would seem to be a lot scarcer these days.

References: 
Bourrelly, P. (1964) Une nouvelle coupure générique dans la famille des Desmidiées: le genre Teilingia.
Coesel, P.F.M. & Meesters, K.J. (2023 second edition) Desmids of the Lowlands.
Ralfs, J. (1848) The British Desmidieae.
West, W., West, G.S. & Carter, N. (1923) A Monograph of the British Desmidiaceae, Volume 5.