Phylum: Charophyta   Family: Desmidiaceae

Cosmocladium perissum J. Roy & J.P. Bisset 1894

First described and illustrated by Archer (1894: 254, pl. 2: 14). Cells small, nearly as broad as long, sinus fairly deep and opening widely. Side-view elliptical, chloroplast axile with a single pyrenoid. Archer describes the zygospore as: “large in proportion to the fronds, extremely irregular in shape, and reddish-brown in colour. Grows apparently in free groups.”
Cell dimensions: L: 11-13 µm; B: 10-12 µm; Is: 6-7 µm; Th: 5-7 µm.
“This species never forms very large colonies, and more than 4 cells are scarcely ever found together. The cells occur in a simple row, loosely joined by very delicate gelatinous threads.” – (West, West & Carter 1923: 201). Found in mesotrophic habitats but rarely recorded.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition) Desmids of the Lowlands.
Prescott, W.G. et al. (2001 reprint) A Synopsis of North American Desmids. Part II, Desmidiaceae: Placodermae, Section 3.
Roy, J. & Bisset, J.P. (1894) On Scottish Desmidieae.
West, W., West, G.S. & Carter, N. (1923) A Monograph of the British Desmidiaceae, Volume 5.