Phylum: Charophyta   Family: Desmidiaceae

Staurodesmus semilunaris (W. Schmidle) P.F.M. Coesel & K.J Meesters 2013

First described and illustrated by Schmidle (1896: 26, pl. 16: 9) as Arthrodesmus incus f. semilunaris. It was recently moved to its current genus and raised to species rank by Coesel & Meesters (2013: 34, pl. 15: 15-16). It differs from the similar Staurodesmus phimus in having crescent-shaped semicells and is a little larger.
Cell dimensions: L: 20-30 µm; B: 20-30 µm; Is: c.8 µm; Sp: 6-9 µm; Overall LxB: 35 x 37.
A very rare taxon, not previously recorded from Britain. Found in slightly acidic conditions such as moorland pools and abandoned peat-cuttings.

References: 
Coesel, P.F.M. & Meesters, K.J. (2013).. European Flora of the Desmid Genera Staurastrum and Staurodesmus.
Schmidle, W. (1895).. Beiträge zur alpinen Algenflora.