136 FRESH-WATER ALG.E OF THE UNITED STATES. 



c. variaMliSj Kg. Pale green, cells as long or a little longer, 

 rarely twice as long as wide. Diameter, 5.5-7 yw. 



d. stagnor-unij Bab. Dirty green, in soft, closely inter 

 woven, floating masses ; cells as long or somewhat 

 longer than broad ; membrane very thin. 



e. tenerrima, Kg. Light green, or dull yellow, mucous; 

 separated from var. C. by the brighter green cell 

 contents ; cells quadrangular. 



f. thermarum, Wartm. Articulations one to three diameters, 

 filaments long, tenacious, affects warm water; 

 flumes of waste water from steam engines and the 

 like. 



Plate CXVIII, figs. 9, 10, the typical form; figs. 22-26, 

 an unusual form, found attached to the wooden sides of a 

 trout pond ; many of the cells swollen to double and some- 

 times nearly triple the diameter of the filaments. For 

 something similar see V. varia ; figs. 27, 28, U. flaccida ; figs. 

 11, 12, filament of var. subtilissima ] figs. 13, 14, var. stag- 

 norum-, figs. 15, 16, var. va/riabUis] figs. 17, 17, var. tener- 

 rima] figs. 18, 19, var. thermarum. 



ULOTHRIX RIVTJLARIS, Kg. * 



Light green, often constricted at joints; rather crisp; 

 cells equal or half as long as wide ; diameter 9-10 ^ ; mem- 

 brane rather firm. 



Usually found attached to grasses and plants near shores 

 of rivers. 



Plate CXVIII, figs. 6, 7, 8, parts of three filaments. 



Var. cataracta, Wolle. 



Forming bright yellowish green tufts on small grassy 

 water-falls, furnished with branchlets more or less hooked, 

 serving as tendrils to hold their position in the rapid Avaters. 

 Diameter of filament, 10-12 ^ ; articulations one to two times 

 as long as wide. 



Plate CXVIII, figs. 29-33. 



ULOTHRIX COMPACTA, Kg. 



Yellowish green, mucous ; cells shorter than the diameter, 

 which ranges from fe-7.5 //. 



Quiet waters. 

 Plate CXVIII, figs. 20-21, parts of filaments. 



