TAXONOMIC KEY 11 



21A Frustules sigmoid or "S" shaped Gyrosigma 



21B Frustules not sigmoid 22 



22A Frustules longitudinally sym- 

 metrical, other than lunate in 



valve view 25 



22B Frustules with raphe in both valves, 

 longitudinally asymmetrical, 



lunate in valve view 23 



23 A Valves with transverse costae Epithemia 



23B Valves without transverse costae 24 



24A Raphe a smooth curve with well 



defined central and polar nodules Cymbella 



24B Raphe not a smooth curve, gibbose 



with marginal central nodule Amphora 



25A Frustules with raphe in both valves 27 



25B Frustules with pseudoraphe in one valve and 



raphe in other valve 26 



26 A Frustules wedge-shaped in girdle 



view and cuneate in valve view Rhoicosphenia 



26B Frustules shaped otherwise Achnanthes 



27A Raphe extended length of valve; polar 

 nodules evident; central nodules 



lacking Eunotia 



27B Raphe restricted to polar regions 28 



28A Raphe located in a canal Nitzschia 



28B Raphe not located in a canal 29 



29A Frustules with symmetrical valves 30 



29B Frustules with valves longitudinally sym- 

 metrical but transversely asym- 

 metrical Gomphonema 



30A Valves with transverse costae Pinnularia 



30B Valves with transverse punctae Navicula 



31A Cells solitary 45 



31B Cells colonial or grouped 32 



32A Cells enclosed in conical to cylin- 

 drical lorica; joined lorica have 

 treelike appearance Dinobryon 



