13. Frustules with transverse septa which show as bands across the cell 

 in valve view; raphe in a canal within a marginal keel, the canal 



with pores Denticula 



PI. 5, Fig. 11 



13. Frustules without transverse septa; raphe in a central axis of the 

 valve; a prominent central nodule which extends both directions on 

 either side of the raphe, the valve with costae Diploneis 



PI. 5, Fig. 13 

 14(1). Frustules bilaterally undulate in valve view, the poles capitate; 

 in girdle view rectangular, the septa showing as inward projecting 



processes extending to the intercalary bands Terpsinoe 



PI. 5, Fig. 10 



14. Frustules shaped otherwise, without septa, or, if present, not so 

 arranged l^ 



15. Frustules with one or two spine-like extensions at the poles 16 



15. Frustules without spines at the poles 18 



16. Frustules with many intercalary bands; cells solitary 17 



16. Frustules without intercalary bands; cells arranged in filaments Melosira 



PI. 6, Fig. 4 



17. Frustules rectangular in girdle view, with two spines at each pole Attheya 



PI. 6, Fig. 2 



17. Frustules extended into a single spine at each pole; wall markings 

 usually lacking Rhizosolenia 



PI. 5, Fig. 6 



18. Frustules cylindrical in girdle view (quinine capsule-shaped), at- 

 tached end to end in filaments; polar margins often with denticula- 

 (•jQjis Melosira 



PI. 6, Fig. 4 



18. Frustules not cylindrical, not attached in filaments 19 



19. Frustules triangularly divided ( 3-parted ) with a pseudoraphe in each 

 valve; frustules non-septate Centronella 



PI. 7, Fig. 16 

 (According to some students this is a questionable genus of Diatoms) 



19. Frustules not triangularly divided 20 



20. Frustules without a raphe in valves; pseudoraphe showing in both 

 valves 21 



20. Frustules with a raphe in at least one valve 30 



21. Frustules in girdle view elongate-rectangular, forming a circular 

 colony in which the cells radiate from a common center like spokes 



of a wheel, in valve view sUghtly enlarged at the poles Asterionella 



Pi. 6, Fig. 6 



21. Frustules shaped and arranged otherwise 22 



22. Frustules wedge-shaped in girdle view, adjoined side by side to 

 form flat, circular, semicircular or fan-shaped colonies (sometimes 

 spiral bands or ribbons) Meridion 



PI. 6, Fig. 3 



22. Frustules other shapes, or without fan-like arrangement 23 



23. Frustules shghtly arcuate or bent in the longitudinal axis 24 



23. Frustules not arcuate 25 



[944] 



