Phylum: Charophyta   Family: Desmidiaceae

Cosmarium pericymatium C.F.O. Nordstedt 1875

A medium-sized cell, with a shallow, widely gaping sinus. Semicells sub-orbicular with a rounded apex, the smooth wall is covered with fine pores and has 12-16 undulations. Chloroplast axile, furcoid, with one pyrenoid.
Cell dimensions: L: 39.3-50 µm, B: 24.4-30.5 µm, Is: 18.5-24.2 µm; Th: 21.2-24.9 µm, L/B: 1.61-1.74.
A rare species, seemingly from habitats that tend to dry out periodically such as ephemeral pools and wet rocks. West & West (1908: 34) note: “The typical plant is not known to occur in the British Islands.”

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium. Part 1: Text. Part 2: Plates.
West, W. & West, G.S. (1908). A Monograph of the British Desmidiaceae, Volume 3.