Phylum: Charophyta   Family: Desmidiaceae

Staurodesmus mucronatus var. subtriangularis (W. & G.S. West) H.T. Croasdale 1957

First described and illustrated by West & West (1903: 545, pl. 17: 11) as Staurastrum mucronatum var. subtriangulare. It was later moved to its current genus by Croasdale (1957: 132, pl. 2: 38). It differs from the nominate variety in have more triangular semicells, hence the varietal name. It is also larger with a broader isthmus.
Cell dimensions: L: 20-26 µm; B: 20-26 µm; Is: 10-11 µm; Sp: 1.5-3.5 µm; Overall B: 25-31.
Similar habitat to the nominate variety and just as scarce.

References: 
Coesel, P.F.M. & Meesters, K.J. (2013). European Flora of the Desmid Genera Staurastrum and Staurodesmus.
Croasdale, H.T. (1957). Freshwater algae of Alaska I. Some desmids from the Interior. Part 3: Cosmariae concluded.
West, W. & West, G.S. (1903). Scottish freshwater plankton: I.
West, W., West, G.S. & Carter, N. (1923). A Monograph of the British Desmidiaceae, Volume 5.