Phylum: Charophyta   Family: Desmidiaceae

Cosmarium tetragonum (C.W. von Nägeli) W. Archer 1861

Cells medium-sized, deeply constricted with a linear sinus. Semicells with a quadrate lower portion then attenuating slightly towards the apex. Basal angles rounded, lateral sides with two sub-lobes, upper one often sub-papillate. Apical angles rounded, apex with two raised points near the centre resulting in an apical shallow notch. Cell wall in face view thickened, smooth with a narrow furrow at some distance from the lateral and apical margin, emanating from close to the basal angles, with three branches: two pointing towards the apical angles and one pointing towards the centre of the apex (only visible in empty cells). A supra-isthmial granule is usually present, quite pronounced in a population from North Tolsta, Lewis; the subelliptic lateral view shows them well. Chloroplast axile with one pyrenoid. Zygospore unknown.
Cell dimensions: L: 45-47.2 µm; B: 24.4-29.3 µm; Is: 9.2-11.2 µm; Th: 19-20 µm; L/B: 1.61-1.85.
A widespread but rare desmid found in mesotrophic, boggy or marshy habitats, but also with moss on wet rocks.

References: 
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.