Phylum: Charophyta   Family: Closteriaceae

Closterium jenneri var. curvatissimum (West & G.S. West) A.J. Brook & D.B. Williamson 2010

This variety was originally named by the Wests in 1903 after a single find in a loch in the Outer Hebrides. Then it was associated with Closterium cynthia. This single cell had a curvature of 210 degrees with the apices slightly incurved. Prescott et al. (1975) discuss a number of cells found across North America, Alaska and Finland with a curvature range of 203-210 degrees. The image they depict is very similar to the one found here where the apices are not incurved. Coesel & Meesters (2007, Plate 16: figs. 4-5) have less curvature. Figure 5 has an arc of about 160 degrees (fig. 4 looks more like the nominate variety). Clearly a very rare desmid and more finds are needed to clearly define the parameters.
Note: This was a variety of Closterium cynthia until recently. One of the defining characters separating the closely related cynthia & jenneri is the lack of striations in jenneri, but more than one author goes on to suggest jenneri can display faint striae. A rather tenuous distinction in my opinion. Considering the Wests original find, mine and others support striae, it would probably be better left with Cl. cynthia.

References: 
Brook, A.J. & Williamson, D.B. (2010) A Monograph on some British Desmids.
Prescott, W.G., et al. (2001 reprint) A Synopsis of North American Desmids. Part II, Desmidiaceae: Placodermae, Section 1.
West, W. & West, G.S. (1904) A Monograph of the British Desmidiaceae, Volume 1
Coesel, P.F.M. & Meesters, K.J. (2007) Mesotaeniaceae and Desmidiaceae of the European Lowlands