Phylum: Charophyta   Family: Desmidiaceae

Cosmarium margaritiferum J. Ralfs 1848

First described by Ralfs 1848: 100, pl. 16: 2; pl. 33: 6) and illustrated by Jenner. It can often sport a large central papilla surrounded by a gradation of granules, the disposition of which is extremely variable. In apical and side view they show as a protuberance. Similar in size and shape to Cosmarium kirchneri and Cosmarium luxuriosum but those species have defining characters.
Cell dimensions: L: 49-61 µm; B: 42-56 µm; Is: 13-18 µm; Th: 28-36 µm; L/B: 1.09-1.17.
Zygospore dimensions: 52-68 µm.
A common desmid from vegetated loch-margins and moorland pools, and occasionally with Sphagnum. It prefers less acidic and more mesotrophic conditions.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
Kouwets, F.A.C. (in manuscript) European Flora of the Desmid Genus Cosmarium.
Ralfs, J. (1848). The British Desmidieae.
West, W. & West, G.S (1908). A Monograph of the British Desmidiaceae, Volume 3.