Phylum: Charophyta   Family: Desmidiaceae

Staurastrum cerastes P. Lundell 1871

First described by Lundell (1871, p. 69, pl. 4: 6) from near Uppsala, Sweden. West, West & Carter (1923, p. 141, pl. 150: 16 & 151: 1) note a few sites in Wales, Scotland and Ireland suggesting an Atlantic distribution. It occurs as 3- and 4-radiate cells as seen in apical view.
Cell dimensions: L: 50-60 µm; B: 60-70 µm; Is: 10-12.5 µm.
It's a rare desmid of slightly acidic habitats.

References: 
Coesel, P.F.M. & Meesters, K.J. (2013) European Flora of the Desmid Genera Staurastrum and Staurodesmus.
Lundell, P.M. (1871) De Desmidiaceis, quae in Suecia inventae sunt, observationes criticae.
West, W., West, G.S. & Carter, N. (1923) A Monograph of the British Desmidiaceae, Volume 5.