Phylum: Charophyta   Family: Desmidiaceae

Cosmarium tetragonum (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 the Tolsta population; the subelliptic lateral view shows them well. Chloroplast axile with one pyrenoid. Zygospore unknown. A widespread but rare desmid found in mesotrophic, boggy or marshy habitats, but also with moss on wet rocks.
Cell dimensions: L: 38-50.5 µm, B: 22-29.3 µm, Is: 7-11.2 µm; Th: 17-20 µm, L/B: 1.61-1.87.

References: 
Archer, W. [In Pritchard, A. (1861 fourth enlarged edition) A history of infusoria, including the Desmidiaceae and Diatomaceae, British and foreign.]
Kouwets, F.A.C. (in manuscript) European Flora of the Desmid Genus Cosmarium.
West, W. & West, G.S. (1908) A Monograph of the British Desmidiaceae, Volume 3.