254 OSCILLATOREJ2. 



16. OSCILLATORIA VIOLACEA Johmt. 



Plate LXXII. Fig. 10. 



Char. " Mass gelatinous, dark purple. Filaments very 

 slender, straight, without perceptible, transverse striae, laid 

 on a thin, compact, greenish substratum." Johnston. 



O. violacea Johnston, Berw. Fl. p. 264. ; Harv. in Hook. 

 Br. Flor. p. 377. ; Harv. in Manual, p. 166. Oscillatoria 

 rubiginosa Carmichael, MS. 



Hob. Rapid streams near Berwick upon Tweed: Dr. 

 Johnston. On stones in the bottoms of rivers, co. An- 

 trim : D. Moore. Appin : Capt. Carmichael. 



When dry, the filaments of this species assume a blackish 

 green cast, with an evident gloss. The filaments are thicker 

 than those of O. subfusca, but do not, like those of that spe- 

 cies, preserve their calibre in drying : they are also more 

 brittle. Striae in the dried specimens almost invisible, at a 

 distance usually of about two diameters from each other. 

 Dr. Johnston considers this to be the " Conferva mucosa con- 

 fragosa rivulis innascens " of Dillenius. 



17. OSCILLATORIA RUPESTRIS Ag. 

 Plate LXXII. Fig. 11. 



Char. Stratum blackish green, thick, opaque, extremely tough. 

 Filaments rigid, brittle. Strife almost imperceptible, at 

 distances of about half a diameter. 



Ag. Syst. p. 63. ; Harv. Hook. Brit. Fl. p. 377. ; Harv. in 

 Manual, p. 166. O. tenax Carm. MS. 



Hob. On the precipitous face of cascades. Appin : Capt. 

 Carmichael. 



" Stratum extensive, slimy, remarkably tough and elastic, 

 black on the surface, ash-coloured underneath: when dry 

 blackish green. Filaments pale green, straight, or variously 

 curved, radiating, but not equally in all directions." Carm. 

 MS. The dried stratum of this species resembles in colour 



