Phylum: Charophyta   Family: Desmidiaceae

Staurastrum cristatum var. oligacanthum (L.A. de Brébisson ex W. Archer) P.F.M. Coesel & K.J. Meesters 2013

First described by Brébisson in Archer (1866: 189) as Staurastrum oligacanthum. This was a perfectly acceptable species for almost 150 years then, inexplicably, Coesel & Meesters (2013: 88, pl. 61: 6-8) made it a variety of Staurastrum cristatum. In my opinion, the taxon is sufficiently distinct from S. cristatum to justify its original status.
In West, West & Carter (2023: 49) they paraphrase Archer (l.c.) "In St. cristatum the semicells are elliptic fusiform, both ventral and dorsal margins are convex, and there is a mucronate angle on each side (usually submamillate). In St. oligacanthum the semicells are angular and roughly hexagonal with a markedly truncate apex. In ventral view the lateral margins of St. cristatum are slightly convex, but in St. oligacanthum distinctly concave."
I would add that in St. cristatum the breadth of the isthmus in no more than 19 µm, in St. oligacanthum it is close to 25 µm.
Cell dimensions: L. 40-50 µm, B. 40-50 µm, Is. c.25 µm.
Found in vegetated loch-margins and pools but a rare taxon.

References: 
Archer, W. (1866). Staurastrum oligacanthum and Staurastrum cristatum exhibited.
Coesel, P.F.M. & Meesters,K.J. (2013). European Flora of the Desmid Genera Staurastrum and Staurodesmus.
West, W., West, G.S. & Carter, N. (1923). A Monograph of the British Desmidiaceae, Volume 5.