Phylum: Charophyta   Family: Desmidiaceae

Cosmarium charnainense C.D.N. Johnson 2023

First described and illustrated by Johnson (2023: 19, figs. 13, 15-17) with a drawing by Frans Kouwets (fig. 14).
Cells small, about as long as broad to marginally broader than long. Sinus slightly open with a rounded apex and parallel sides for half the length, then opening widely to broadly rounded basal angles. Semicells trapezoid-hexagonal with the lower lateral angles divergent and more or less straight. The broadest part is just above the median point. Upper lateral angles acutely convergent and retuse. Apex truncate or slightly retuse. Each semicell has two small granules disposed asymmetrically: in front view the left granule is placed near the left lateral angle whereas the right granule is placed near the right apical angle. In apical view the asymmetry is clearly seen. Side view is orbicular. The cell-wall is smooth with numerous mucilage pores. Chloroplast axile with a single pyrenoid per semicell. The zygospore is subglobose with dark olive-green mesospore and almost black edge, the transparent exospore has the gametangial cells embedded.
Dimensions: L: 14.0–14.7 µm. B: 14.4–15.4 µm. Is: 6.2–6.9 µm. Th: 8.1–8.3 µm. L/B: 0.95–1.01.
Zygospore: 22.3 x 18.7 µm.
Complete taxonomic description: Cosmarium charnainense.

References: 
Johnson, C.D.N. (2023). Descriptions of two previously undescribed species from the Outer Hebrides, Scotland, UK. Desmidiological Communications 10.