Phylum: Charophyta   Family: Desmidiaceae

Spondylosium planum (F. Wolle) W. West & G.S. West 1912

First described and illustrated by Wolle (1884: 29, pl. 4: 3-4) as a variety of Sphaerozosma pulchrum (S. pulchrum var. planum). Later West & West (1912: 430, pl. 19: 5-8) raised it to species level as Spondylosium planum. A filamentous species that can break up into smaller parts, which can then be confused with Cosmarium species.
Cell dimensions: L: 10-20 µm; B: 11-25 µm; Is: 5-11.5 µm; Th: 6-11 µm; L/B: 0.80-0.90.
West, West & Carter (1923: 223) note “It is a common species in the plankton of many British lakes.” In the Outer Hebrides this is a rare species occasionally found in acidic, oligo-mesotrophic, habitats.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition) Desmids of the Lowlands.
West, W. & West, G.S. (1912) On the periodicity of the phytoplankton of some British Lakes.
West, W., West, G.S. & Carter, N. (1923) A Monograph of the British Desmidiaceae, Volume 5.
Wolle, F. (1884) Desmids of the United States and list of American Pediastrums with eleven hundred illustrations on fifty-three colored plates.