24. Central smooth area present, extending to ventral (concave) margin 

 as seen in valve view, the ventral margin with a slight swelling in 



the midregion Ceratoneis 



PI. 6, Fig. 7 



24. Central smooth area lacking; pseudoraphe narrow throughout length 

 of valve, margins showing sharply pointed undulations in valve 

 view Amphicampa 



PI. 6, Fig. 5 



25. Frustules attached in zig-zag chains (sometimes semi-stellate or 

 radiate); longitudinal septa present, straight; rows of transverse 

 puncta visible in valve view; frustules not showing transverse cos- 



tae Tabellaria 



Fig. 1, p. 949; PI. 6, Fig. 11 



25. Frustules not arranged in zig-zag chains, or if so, with curved septa 26 



26. Frustules with curved septa; costae present, appearing as septa; frus- 

 tules arranged in bands (sometimes in zig-zag chains) Tetractjclus 



PL 6, Fig. 10 



26. Fmstules without septa 27 



27. Frustules with prominent costae in the valves 28 



27. Frustules without prominent costae 29 



28. Valve view symmetrical, usually elliptic or subcylindric, often with 

 subcapitate poles; in valve view with a faint pseudoraphe; girdle 

 view rectangular Diatoma 



PI. 6, Fig. 8 



28. Valve view symmetrical, egg-shaped; asymmetrical and wedge-shaped 



in girdle view; transverse costae conspicuous Opephora 



PI. 6, Fig. 15 



29. Frustules quadrate or rectangular in girdle view, attached side by 

 side to form ribbons ( rarely in chains ) ; valve view fusiform, the 

 poles narrowed from an enlarged central region Fragilaria 



Fig. 2, p. 949; PI. 6, Fig. 14 



29. Frustules elongate and straight (rarely slightly curved), needle- 

 shaped in both views, or with slightly capitate poles; pseudoraphe 

 between transverse striae; frustules solitary or in radiating colonies, 

 attached to substrate, singly or in clumps, at one end by short 

 gelatinous stalks Synedra 



PI. 6, Fig. 9 



30. Frustules lunate or slightly curved in valve view; rectangular or boat- 

 shaped in girdle view 31 



30. Frustules some other shape in valve view 34 



31. Curvature slight (frustules often nearly straight); frustules bearing 

 a keel near the margin of a valve in which the raphe is inclosed, 

 the location of the keel marked by a row of dots; frustule quadrangu- 

 lar in cross section Hantzschia 



PL 7, Fig. 1 

 31. Curvature decidedly evident; frustules not bearing a keel 



on the valve; asymmetrical in longitudinal axis; the raphe usually 

 lying much closer to the ventral (concave) margin 32 



[945] 



