Phylum: Charophyta   Family: Desmidiacea

Cosmarium reniforme var. compressum  C.F.O. Nordstedt 1887, sensu P.F.M. Coesel 1979

When Nordstedt (1887) described var. compressum he was referring to compressed sides when viewed apically. Many later authors interpreted this incorrectly as referring to compressed cells in face-view. This particular variety is distinguished by having retuse apices and a broader isthmus than the nominate variety. Coesel’s (1979: 391, pl. 17: 6) dimensions, calculated from his drawing, are remarkably similar to mine.
Cell dimensions: L: 52-54 µm; B: 49-50.1 µm; Is: 20.3-22.3 µm; Th: 29.7 µm; L/B: 1.06-1.08.
Coesel’s dimensions: L: 54.5 µm; B: 51.5 µm; Is: 21.5 µm; L/B: 1.06.
Because of the taxonomic confusion over the Cosmarium reniforme complex, distribution is unknown in the Outer Hebrides. Current finds have been in mesotrophic pools.

References: 
Coesel, P.F.M. (1979). Desmids of the broads area of N.W.-Overijssel (The Netherlands) II.
Kouwets, F.A.C. (in manuscript). European Flora of the Desmid Genus Cosmarium.