Phylum: Charopyta   Family: Desmidiaceae

Staurodesmus extensus var. joshuae (R. Gutwinski) E. Teiling 1967

Predominantly a biradiated species with a slightly extended isthmus giving a broad rounded extremity to the sinuses. Semicells are subrectangular to triangular with long convergent to parallel spines from the apical-lateral angles.
Cell dimensions (2): L: 20.9-21.3 µm; B: 21.3-21.7 µm; Is: 6.4-6.7 µm; Sp: 10-13 µm; Overall B: 47-48.
Found in acidic moorland pools and Sphagnum bogs. The scarcest of the varieties and of little-known status in the Outer Hebrides.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
Teiling, E. (1967). The desmid genus Staurodesmus. A taxonomic study.
Van Westen, M.C. (2024). Sieralgen in Drenthe.