10 ALGAE IN PHYTOPLANKTON OF THE UNITED STATES 



9A Heterocysts terminal Cylindrospermum 



9B Heterocysts intercalary Ahphanizomenon 



10A Cell walls without punctae or 



striae 31 



10B Cell walls rigid, ornamented with 

 punctae or striae, indestructible 



by clearing techniques 11 



11A Frustules adiametric, two or more times 



longer than wide, generally elongate 15 



11B Frustules isodiametric, generally shorter in 

 length than in diameter, round or ellip- 

 tical or ovoid or nearly so 12 



12A Frustules elliptical or ovoid or 



nearly so 14 



12B Frustules discoid or nearly so 13 



13A Valves radially punctate Stephanodiscus 



13B Valves with two concentric regions, the in- 

 ner being smooth Cyclotella 



14A Frustules with marginal keel 



containing a raphe Surirella 



14B Frustules with a pseudoraphe or 

 with a raphe not in a marginal 



keel Cocconeis 



15A Frustules cylindrical arranged end to end 



into filament Melosira 



15B Frustules not arranged into filaments 16 



16A Frustules with a raphe in at least 



one valve 21 



16B Frustules without a raphe in either 



valve, pseudoraphe evident 17 



17A Frustules united in zigzag chains Tabellaria 



17B Frustules not in zigzag chains 18 



18A Frustules united laterally Fragilaria 



18B Frustules not united laterally 19 



19A Frustules united apically forming spokelike 



colony Asterionella 



19B Frustules not forming spokelike colony 20 



20A Frustules needle shaped without 



costae Synedra 



20B Frustules with prominant costae Diatoma 



