1^ 



3^a Central area extending laterally to the margins of the 

 valve, striae absent along lateral margins of the 

 central area Stauronei s (p. 70) 



3*)-b Central area not extending to the margins of the valve, 

 striae present along lateral margins of the central 

 area Navicula (p. 71) 



35a Keel elevated into a lateral "wing" or flattened on 



the valve surf ace 37 



35b Keel elevated into an axial "wing" extending along the 



central axis of the valve 36 



36a Keel sigmoid, usually seen in girdle view (hour-glass- 

 shaped), frustule twisted along the longitudinal axis; 



girdle broad with many longitudinal folds 



Amphiprora (p. 80) 



36b Keel not sigmoid, girdle simple, not folded, keel 



eccentric Tropidoneis (p. 82) 



37& Valves with numerous internal transverse ribs extending 



completely across the valve 38 



37b Valves without internal transverse ribs h-0 



38a Raphe and axial area with "V"-shaped medial extension; 



with transverse septae appearing as costae and alternat- 

 ing with two or more rows of punctae 



Epithemia (p. 83) 



38b Raphe and axial area without a "V"-shaped medial 



extens ion 39 



