ANALYSIS OF THE GENERA. \) 



15. Frustules adherent Cocconeis. 



„ free 16 



16. Valves transversely undulated Cymatopleura. 



„ without undulations 17 



1 7. Valves with longitudinal ridges 18 



,, plane or convex 20 



18. Valves with a central ridge or keel 19 



,, with two or more ridges Amphipleura. 



19. Valves elliptical, deeply constricted Amphiprora. 



„ much elongated ; constriction, if any, slight Nitzschia. 



20. Valves sigmoid Pleurosigma, 



„ straight 21 



2 1 . Valves with central nodule 22 



„ -with central nodule dilated into a Stauros Stauroneis. 



22. Valves striated Navicula. 



„ COState PlNNULARIA. 



23. Frustule linear or much elongated Synedra. 



„ elliptical or cymbiform 24 



,, cuneate 25 



24. Valves having terminal and subcentral nodules Cocconema. 



„ without nodules Doryphora. 



25. Valves with a nodule in the centre and at each end. . . . Gomphonema. 



„ without nodules 26 



26. Stipes obsolete or very short Podosphenia. 



,, elongated 27 



27. Stipes filiform Rhipidophora. 



„ incrassate Licmophora. 



28. Filament compressed, continuous 29 



„ a zigzag chain 39 



„ cylindrical 44 



29. Filament spiral Meridion. 



„ coiled Bacillaria. 



„ straight 30 



30. Frustule with internal septa 37 



„ vdthout septa 31 



31. Valves with central and terminal nodules Diadesmis. 



,, without nodules 32 



32. Frustule curved on Front View Achnanthes. 



„ oblong on Front View 33 



