241. Cells broadly fusiform in outline, with a long or short anterior 

 horn and 2 or 3 posterior horns; wall heavy and reticulate, 

 composed of plates (sometimes not easily discerned).. Ceratium 



241. Cells not broadly fusiform and without prominent anterior 



and posterior horns 242 



242. Cells with a definite membrane but without a true wall; not 

 showing plates; possessing a transverse furrow which completely 

 encircles the cell Gymnodinium 



242. Cells with a definite wall which usually shows plates; with a 

 transverse furrow sometimes completely encircling the cell 243 



243. Wall heavy, plates easily discerned; transverse furrow completely 

 encircling the cell; cell very little flattened dorsiventrally 245 



243. Wall thin, plates often seen with difficulty; transverse furrow 

 complete or not; cell sometimes flattened dorsiventrally ....244 



244. Cells strongly flattened dorisventrally (PI. 90, Figs. 4-6); plates 

 scarcely showing; transverse furrow incompletely encircling 



the cell Hemidinium 



244. Cell little or not at all flattened dorsiventrally; plates readily dis- 

 cernible; transverse furrow completely encircling the cell Glenodinium 



245. Cells not flattened dorsiventrally, with 1 antapical plate (plate 



at the posterior pole) '^Gonyaulax 



245. Cells with 2 antapical plates, slightly flattened dorsiventrally... Peridinium 



246. Colony actively swimming by means of flagella .248 



246. Colony non-motile, or cells with rhizopodal processes 247 



247. Cells 2, or 4-8, irregularly arranged in a common mucilage Chrysocapsa 



247. Cells 16 (rarely 32) radially arranged in a wide gela- 

 tinous sheath which is heavily impregnated with granular 



waste products Chrysostephanosphaera 



248. Colony spherical, composed of ovoid or pyriform cells arranged 



at the periphery of a colonial mucilage - 249 



248. Plant not a spherical colony of ovoid or pyriform cells 251 



249. Cells each with 2 apical cylindrical receptacles from which a 



long silicious rod extends Chrysosphaerella 



249. Cells not bearing silicious rods 250 



250. Cells elongate-ellipsoid, or elongate-pyriform, with silicious scales 



in the apical region; swimming by 2 flagella of equal length Synura 



250. Cells small,ovoid or pyriform, without apical silicious scales; 

 flagella 2, unequal in length Uroglenopsis 



251. Cells amoeboid, without loricas, adjoined in loose linear series 



by their long, thin pseudopodia Chrysidiastrum 



251. Cells with vase-shaped or conical loricas, forming arbuscular 

 or diverging colonies, with 1 or 2 loricas emerging from the 



mouth of the lorica below Dinobryon (in part) 



252. Plant an attached parenchymatous mass in which filamentous 

 arrangement of cells is not clearly evident; endospores formed in 



the upper or outer cells of the plant mass, or of the hranches...." Pleurocapsa 



252. Plants otherwise, not a parenchymatous mass 253 



253. Plants unicellular or colonial, usually with an evident sheath, 



which may be lamellated -. 254 



253. Plants filamentous; cells arranged in 1 or more series and in- 

 closed or not inclosed by a sheath (PI. 112, Figs. 1—8). (The 

 series of cells is termed a trichome and this, together with its 

 sheath, is called a filament in this group of the algae.).. 274 



[590] 



