191. Walls of cells often thick, formed of 2 pieces which usually show 



an overlapping in the midregion of cylindrical cells Tribonema 



191. Walls of cells thin, the 2 sections of the wall scarcely showing 



an overlapping; cells quadrate Bumilleria 



192. Plant a small, green vesicle, 1-2 mm. in diameter, forming 

 gregarious patches on moist soil Botrydiuin 



192. Plant not a green vesicle. 19'^ 



193. Cells solitary or incidentally clustered 194 



193. Cells in definite or indefinite colonies, 2 or jnore cells inclosed 



in common mucilage, in clumps, or in stalked families.— 219 



194. Cells attached, sessile or stipitate 212 



194. Cells swimming, or free-floating— --- 19^ 



195. Cells swimming by 2 flagella of unequal length Chlorochromonas 



195. Cells free-floating, planktonic or intermingled with other algae 196 



196. Cell walls smooth 197 



196. Cell walls sculptured with reticulations, depressions, or with spines -204 



197. Cells spherical 19^ 



197. Cells fusiform, cylindrical, or crescent-shaped- 200 



198. Cells inclosed in a gelatinous sheath (usually in families of 2-16, 

 sometimes solitary) Chlorobotnjs (in part) 



198. Cells not inclosed in a gelatinous sheath 199 



199. Wall in 2 sections, separating and persisting as hemispherical 

 membranes near liberated autospores Diachros (in part) 



199. Wall not in 2 sections; mother cell breaking down irregularly 



to liberate autospores Botrydiopsis (in part) 



200. Cells elongate-cylindric, coiled or S-shaped, both poles sym- 

 metrically and broadly rounded Ophiocytium (in part) 



200. Cells oblong or short-cylindric, or fusiform, the poles sometimes 



not symmetrical, curved but not coiled 201 



201. Cells oblong, short-cylindric, IV2-2 times as long as broad, lateral 



walls parallel, poles symmetrically rounded Monallantus 



201. Cells fusiform or irregularly cylindrical; poles unsymmetrical.. 202 



202. Cells fusiform - ---- 203 



202. Cells irregularly cylindrical, usually narrowed at the poles, 

 which are unsymmetrical; chromatophores parietal discs, usually 

 numerous - Bumilleriopsis 



203. Cells broadly fusiform, unsymmetrically and rather abruptly 

 narrowed at the poles - - Pleurogaster (in part) 



203. Cells narrowly fusiform, much bent, sickle-shaped, or lunate Chlorocloster 



204. Cells elongate-cyhndric, with a long spine at 1 or both poles — ._ _— 203 



204. Cells short-cylindric, spherical, triangular, or tetragonal 206 



205. Cells straight or only shghtly bent - - - - - Centritractus 



205. Cells coiled, S-shaped, or hooked at one end Ophiocytium ( in part ) 



206. Cells spherical - - - - 207 



206. Cells shaped otherwise 203 



207. Cell wall sculptured with broad depressions, serrate at the 

 margin - Arachnochloris 



207. Cell wall with straight or curved spines or thread-like appen- 



(jages Meringosphaera 



208. Cells broadly fusiform, abruptly narrowed to form a spine-like 



point at 1 or both poles Pleurogaster (in part) 



[587] 



