266. Cells spherical or ovate, sometimes elliptical; not cylindrical, 



but very little longer than wide 267 



267. Cells round, embedded in copious mucilage with concentric 

 lamellations; individual cell sheaths becoming confluent; plant 

 usually formed by large numbers of cells; occurring mostly in 



aerial habitats, sometimes aquatic Gloeocapsa 



267. Cells of different shapes, ovate, ellipsoid, or, if spherical, not 

 forming shapeless gelatinous masses on moist substrates 268 



268. Cells round, ovate, dividing in 2 planes only to form flat, usually 

 rectangular, plates, with margins of plates sometimes rolled — 269 



268. Cells dividing in 3 planes to form spherical, ovate, or amor- 

 phous masses.— 270 



269. Cells regularly arranged in rectilinear series forming rectan- 

 gular plates Merismopedia 



269. Cells irregularly arranged in 2's and 4's but not in rectilinear 



series Holopedium 



270. Colony hollow, the cells arranged in one layer near the periphery.— 271 



270. Colony not hoUow, the cells distributed throughout the colonial 

 mucilage... 272 



271. Cells at the ends of branching, gelatinous strands which radiate 



from the center of the colony (sometimes scarcely visible). .Gomphosphaeria 



271. Colonies without radiating gelatinous strands Coelosphaerium 



272. Cells longer than wide, distributed throughout the colonial muci- 

 lage, with or without individual cell sheaths apparent ..( Anacysfis, in part) 



Including: a. Aphanothece, with individual cell sheaths not apparent; 

 b. Gloeothece, with individual cell sheaths apparent. 



272. Cells globose 273 



273. Cells crowded, usually very dense, not evenly distributed within 

 the colonial mucilage; pseudovacuoles present or absent (PI. 



102, Fig. 1 ) Microcystis 



(Polycystis) 



273. Cells evenly and relatively remotely spaced throughout the 

 colonial mucilage; cell contents granular or homogeneous, with- 

 out pseudovacuoles ....Aphanocapsa 



274. Heterocysts present, although sometimes infrequent 290 



274. Heterocysts not present; trichome with or without a sheath, 

 sometimes more than 1 trichome vWthin a sheath 275 



275. Trichomes spirally coiled 276 



275. Trichomes straight, bent, or entangled, but not regularly coiled 278 



276. Trichomes with a sheath... Lynghya (in part) 



276. Trichomes without a sheath 277 



277. Plant a 1-celled trichome, up to 300m long Spirulina (in part) 



277. Plant a many-celled trichome, the cross walls indistinct; usually 



much longer than above Arthrospira 



278. Trichomes aggregated, tapering to a point from an attached 



basal prostrate cushion of cells Amphithrix (in part) 



278. Trichomes not tapered, or if tapered, without a basal prostrate 

 cushion of cells 279 



279. Trichomes without a sheath (though hormogonia may show 



a thin sheath ) Oscillatoria 



279. Trichomes with a sheath which is usually very definite 280 



280. Trichomes tapering from base to apex, often ending in a hair 281 



280. Trichomes not tapering, the same diameter throughout their length ...282 



[592] 



