Desmidiacece 



173 



Breb., St. Kjellmanii Wille (fig. 51 E), St. acarides Nordst. and St. Arnellii 

 Boldt, are principally confined to mountainous regions ; others, such as 

 St. pelagicum W. & G. S. West, St. pseudopelagicum W. & G. S. West, 

 St. jacuLiferum West, St. paradoxum Meyen var. longipes Nordst., St. brevi- 

 spinum Breb. and forms of St. anatinum Gke. & Wills (fig. 65 A and B), are 

 abundant in the plankton of lakes. A few of the most beautiful species of 

 the genus, amongst which may be mentioned St. Opkiura Lund., St. Cerastes 

 Lund., St. Arctiscon (Ehrenb.) Lund., St. verticillatuin Arch, and St. longi- 

 spinum (Bail.) Arch., are confined to the extreme western districts of Scotland, 

 Wales and Ireland, and are most abundant in the plankton of those areas. 



Genus Cosmocladium Breb., 1856. The cells of this genus 

 are similar to those of some of the smooth species of Cosmarium, 

 but the individual cells are united by relatively thin mucilaginous 

 threads into branched colonies. Sometimes the entire colony is 



Fig. 66. A, Cosmocladium constrictum (Arch.) Josh., from Pilmoor, N. Yorks. ( x 475). 



B, C. pulchellum Breb., from near Tarbert, Harris, Outer Hebrides (x475). 



C, zygospore of C. perissum Roy & Biss., from the Clova Mts., Forfar ( x 475). 

 D F, Oocardium stratum Nag., after Liitkemiiller ( x 730). 



enveloped in a mucilaginous mass of much less density than the 

 connecting threads. There is one chloroplast in each semicell 

 containing a single pyrenoid. The zygospores are globose and 

 smooth, or they may be lobed (as in C. perissum Roy & Biss.). 



