41. Girdle view bent, wedge-shaped with poles usually truncate, at- 

 tached by stalks or mucilage plugs to substrates; vale view cun- 



eate Rhoicosphenia 



PI. 6, Fig. 18 



41. Girdle view not wedge-shaped 42 



42. Frustules broadly eUiptic or oval, incompletely septate; epiphytic, 

 appressed valve down to substrate; pseudoraphe in epipvalve; raphe 



in hpyovalve Cocconeis 



PI. 5, Fig. 12 



42. Frustules narrowly elliptic, with smooth or undulate margins in 

 valve view; bent, rectangular or naviculate in girdle view; without 

 septa; attached by stalks to filamentous algae and aquatic 



plants Achnanthes 



PI. 6, Fig. 19 



43. Raphe confined to polar regions 44 



43. Raphe evident throughout the length of the valve 45 



44. Frustule straight in the apex; margin smooth in valve view; costae 



on each lateral margin Peronia 



PI. 7, Fig. 17 



44. Frustules bent or curved in the apex; often wavy or imdulate on one 

 margin as seen in valve view; raphe marginal; transversely 



striate Eunotia 



Pi. 6, Fig. 22 



45. Raphe located in a canal 46 



45. Raphe not located in a canal 51 



46. Frustules spirally twisted, raphe accordingly spiral; cells with a 

 keel Cylindrotheca 



PI. 7, Fig. 13 



46. Frustules not spirally twisted 47 



47. Raphe not in a keel 48 



47. Raphe in a keel or wing 49 



48. Raphe in a canal without pores; frustule broadest on the girdle side 

 and seen lying in this position; a clear area in the girdle zone bord- 

 ered by prominent costae, margins usually with a swelling in the 

 midregion as seen in girdle view Rhopalodia 



PI. 6, Fig. 20 



48. Raphe in a canal with pores; lanceolate or elliptic in valve 

 view Denticula 



PI. 6, Fig. 17 



49. Keels on margins of valves opposite one another; quadrangular in 

 cross section Hantzschia 



PI. 7, Fig. 1 



49. Keels on alternate margins of valves 50 



50. Frustules occurring in colonies; keel central Bacillaria 



PI. 7, Fig. 9 



50. Frustules solitary; keel eccentric, diagonally opposite one another; 

 rhomboid in cross section Nitzschia 



PI. 7, Fig. 2 



51. Frustules asymmetrical in either the transverse or the longitudinal 

 axis; key- or wedge-shaped in valve view, shghtly larger at one end 

 than the other 52 



51. Frustules symmetrical in both axes 53 



[947] 



