Phylum: Charophyta   Family: Desmidiaceae

Staurodesmus incus var. indentatus (W. & G.S. West) P.F.M. Coesel 2007

A biradiated species with an extended isthmus with the lateral sides of the semicells indented. This creates a subtriangular shape. Long spines appear at the lateral-apical angles and are predominantly divergent. Compare with Std. extensus (O. Borge) E. Teiling 1948, which also has an extended isthmus but lacks the indented lateral sides to the semicells. Also, the spines are usually shorter.
Cell dimensions: L: 26-37 µm; B: 20-27 µm; Is: 6.4-10 µm; Sp: 22-26 µm; Overall LxB: 42-57 x 57-72.
A rare taxon with an Atlantic distribution. Found in slightly acidic lochs with open water.

References: 
Coesel, P.F.M. (2007). Taxonomic notes on Dutch desmids IV: new species, new names, new combinations.
West, W. & West, G.S. (1912). A Monograph of the British Desmidiaceae, Volume 4.