Phylum: Charophyta   Family: Desmidiaceae

Staurastrum cerastes P. Lundell 1871

An extraordinary desmid with the processes of the semicell's arched to meet in the centre. There are simple dentitions from the processes’ tips extending over the apices where they become bicuspid. There are also bicuspid dentitions at the semicell' bases. It occurs as 3- and 4-radiate cells as seen in apical view.
Cell dimensions (2): L: 53-60 µm; B: 59-62 µm; Is: 11-12 µm; L/B: 0.87-1.01.
West, West & Carter (1923: 141) note a few sites in Wales, Scotland and Ireland suggesting an Atlantic distribution. It's a rare desmid found in acidic habitats.

References: 
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.