Phylum: Charophyta   Family: Closteriaceae

Closterium regulare L.A. de Brébisson 1856

A robust cell with quite coarse striae. The apices are truncately or angularly rounded. West & West (1904: 122) note: “This rare species occupies a position intermediate between Cl. costatum and Cl. stiolatum. It is distinguished from the former by its somewhat smaller size, more attenuated apices, and by the greater number of costae; from the latter it differs in its more attenuated and relatively narrow apices, and by the presence of 11 or 12 visible costae instead of 14 or more striolations.”
Single cell dimensions: L: 268 µm; B: 25 µm; Ap: 9.9 µm; L/B: 11.
It remains rare in the Outer Hebrides. Currently known from a moorland, mesotrophic, loch with marginal vegetation.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
Van Westen, M.C. (2024). Sieralgen in Drenthe.
West, W. & West, G.S. (1904). A Monograph of the British Desmidiaceae, Volume 1.