Phylum: Charophyta   Family: Desmidiaceae

Cosmarium dentiferum var. alpinum E.A. Messikommer 1942

This large cell is broadly rounded with the sinus dilated at the isthmus. The more or less truncate apex has a central dent obvious in most cells. It is covered in small to medium granules with 32-36 visible on the periphery of each semicell. A rare taxon with just one population found on the Outer Hebrides. The sizes of this population are given below: Kouwets’ (in manuscript) are in brackets.
Cell dimensions: L: 60-68 (50-70) µm; B: 53-5-59 (40.5-60) µm; Is: 21-22.5 (15-23) µm; Th: 32-35 (30-35) µm; L/B: 1.13-1.16 (1.16-1.25).
For occurrence, Kouwets gives: “Mesotraphentic, arctic-alpine form, occurring scattered in slightly acidic, boggy and marshy pools, also with Sphagnum, and in quagmire areas along larger mountain lakes; also on wet rocks.” The Outer Hebridean population was from moss on wet rocks.

References: 
Kouwets, F.A.C. (in manuscript). European Flora of the Desmid Genus Cosmarium.