Phylum: Charophyta   Family: Desmidiaceae

Staurodesmus validus var. subincus (W. & G.S. West) P.F.M. Coesel & K.J. Meesters 2013

First described and illustrated by W. & G.S. West (1912: 105, pl. 116: fig. 3) as Arthrodesmus bulnheimii var. subincus. Much later it was moved by Coesel & Meesters (2013: 38, pl. 3: figs 13-17) as a variety of Std. validus. The distinction between this variety and the nominate variety seems to be based almost exclusively on size. Coesel & Meesters (l.c.) give for Std. validus anything greater than L. 60 x B. 55 µm. Anything less than these dimensions is var. subincus. This is fine for fully grown cells but a great many are still developing their spines. In these instances, it is better to measure the parent semicell and its spines and double it for a reliable length. There would seem to be considerable overlap between the two varieties.
Cell dimensions: L: 23-28 µm; B: 20-26 µm.; Is: 7-10 µm; Sp: 14-24 µm.
Infrequently found in acidic waters such plankton of lochs and moorland pools.

References: 
Coesel, P.F.M. & Meesters, K.J. (2013) European Flora of the Desmid Genera Staurastrum and Staurodesmus.
West, W. & West, G.S. (1912) A Monograph of the British Desmidiaceae, Volume 4.