1919] Setchell-Gardner : Myxophyceae 55 



2. Arthrospira breviarticulata S. and G. 



Plate 7, fig. 26; plate 5, fig. 18 



Trichomes very loosely twisted into a more or less regular spiral, 

 and much knotted and contorted, 14.5-16fi diam., comparatively short, 

 slightly torulose, not tapering at the apices; cells 2.5-3. 5)it long; proto- 

 plast pale steel-blue, with few scattered, somewhat angular granules; 

 cell walls thin, distinct, terminal cell very convex, outer wall not 

 thickened. 



Growing on rock.s and among CUidopJiora trichotom-a in tide pools 

 along high-tide level. Lands End, San Francisco, California. 



Setchell and Gardner, in Gardner, New Pac. Coast Alg. Ill, 1918a, 

 p. 466, pi. 39, fig. 18, and pi. 41, fig. 26; Collins, Holden and Setchell, 

 Phyc. Bor.-Amer. (Exsicc), no. 2258. 



Arthrospira hreviarticulata is distinguished from all other species 

 of the genus by its much greater length, and greater diameter, by its 

 being loosely coiled, and by its relatively much shorter cells. It approx- 

 imates the spirally coiled species of Oscillatoria, yet seems to be refer- 

 able rather to Arthrospira. 



15. Spirulina Turp., emend. Gard. 



Trichomes multicellular, cylindrical, evaginate, loosely or tightly 

 coiled into a more or less regular spiral ; apex of trichome usually not 

 tapering ; dissepiments obscured ; terminal cell rounded, without 

 calyptra ; protoplast homogeneous or granular, 



Turpin, in Diet, des sciences nat. de Levrault, vol. 50, 1827, p. 309 ; 

 Gardner, New Pac. Coast Alg. I, 1917, p. 379. 



It has been shown (Gardner, 1917, pp. 377-379) that there is no 

 fundamental difference between the genera Spirulina and Arthrospira, 

 both having multicellular, spirally twisted filaments. We have 

 retained the two genera as a matter of convenience, since the species 

 are few in each genus and well known, and since they fall rather 

 naturally into two groups as regards size. We have retained Spirulina 

 to include the small, more or less tightly coiled forms with inconspicu- 

 ous cross walls, usually described as "unicellular," and Arthrospira 

 to include the large, comparatively short, loosely coiled forms with 

 more or less conspicuous cross walls. 



