15 



39a Raphe canal with pores, valves symmetrical to longitudinal 



axis Denticula (p. 8*4-) 



39h Raphe canal without pores, valves asymmetrical to the 



longitudinal axis Rhopalodia (p. 8^ ) 



40a Valves with lateral keel extending along both margins of 



each valve Vl 



kOb Valves with lateral keel extending along one margin of 



each valve h-2 



^la Valve face longitudinally undulate, undulations 



conspicuous in girdle view; with broad, short, peripheral 



costae; longitudinal pseudoraphe present 



Cymatopleura (p. 86) 



4lb Valve face not longitudinally undulate; valve face with 

 longitudinal folds which appear as heavy costae, folds 

 not conspicuous in girdle view; girdle view rectangular, 

 naviculoid, wedge shaped or sigmoid; valve view linear, 



elliptical, ovate or spirally twisted 



Surirella (p. 87) 



h2a Keels eccentric to the median axis, diagonally opposite; 

 frustules rhombic in cross section, transversely striate, 

 punctate or lacking ornamentation; a row of circular pores 

 ("carinal dots") developed within the keel; frustules 

 usually solitary but occasionally forming irregular ribbon- 

 like chains or stellate colonies Nitzschia (p. 89) 



^2b Keels eccentric to the median axis, directly opposite; 



frustules rectangular in cross section, transversely striate 



or punctate, capitate ends, medianly constructed. . . Hantzschia (p. 98) 



