Desmidiacece 



175 



fig. 67 C) is the largest, and S. excavatum Ralfs (fig. 67 D F) and S. granu- 

 lutum Roy & Biss. are the most widely distributed. 



Genus Onychonema Wallich, 1860. 

 form simple filamentous 

 colonies. The median con- 

 striction is deep and narrow, 

 and the semicells are ellip- 

 tical or reniform, sometimes 

 with strong lateral spines at 

 each side. There are two 

 capitate processes of con- 

 siderable length attached to 

 each apex and disposed asym- 

 metrically. The cells are 

 united into long flexible 

 filaments by the overlapping 

 of these processes over the 

 adjacent cells. There is one 

 axile chloroplast in each 

 semicell, with a single pyre- 

 noid. The zygospores are 

 globose and furnished with 

 simple spines. 



The cells are small and 



There are three British species 

 of the genus, all of which are 

 distinctly rare. The one most 

 generally observed is 0. filiformis 

 (Ehrenb.) Roy & Biss. (length of 

 cells 14 15 M ; breadth 14-5 

 16 x. 



Fig. 67. A, Spondijlosium pulchellum Arch., 

 from Glen Shee, Perthshire (x365). B, S. 

 papillatum W. & G. S. West, from Skipwith 

 Common, E. Yorks. ( x 475). C, Sphcero- 

 zosnia vertebratum Ralfs, from near CrdwalT, 

 Cornwall (x475). D F, Sph. excavatum 

 Ralfs ; D, from Llyn Idwal, N. Wales ( x 475) ; 

 E,zygospore from Puttenham Common, Surrey 

 ( x 475) ; F, zygospore from New Forest, Hants. 

 ( x 475). G H, Onychonema Nordstedtiana 

 Turner, from Strensall Common, N. Yorks. 

 (G, x475; H, x 730). 



Genus Spondylosium Breb., 1844. [Leuronema Wallich, I860.] 

 The cells are small or of medium size and are united by their 

 apices to form filamentous colonies, occasionally twisted and often 

 enveloped in a copious mucus. The median constriction is usually 

 deep and linear and the semicells are of very variable form. The 

 apices are flat or concave and the cells are joined merely by the 

 close apposition of their apices, this being the sole distinguishing 

 feature between Spondylosium and Sphwrozosma. The vertical 

 view is elliptical, triangular, or trilobed. The chloroplasts are as 

 in Sphcsrozosma, and the zygospores are globose and smooth. 



