12 



19a Raphe rudimentary, short, near poles only 



Eunotia (p. 53) 



19b Raphe fully developed, extending the length of the 



valves 20 



20a Valves symmetrical about both the transapical and 



apical plane 2k 



20b Valves asymmetrical about either the apical or 



transapical plane 21 



21a Valves symmetrical about the transapical plane, asymmetrical 



about the apical plane 23 



21b Valves asymmetrical about the transapical plane, 



symmetrical about the apical plane 22 



22a Valve margins with longitudinal "shadow"-lines , 



Gomphoneis (p. 5I4.) 



22b Valve margins lacking longitudinal shadow"-lines 



Gomphonema (p. 5^-) 



23a Valve faces parallel Cymbella (p. 57) 



23b Valve faces not parallel, both valves faces can be seen 



in girdle view Amphora (p. 60) 



2^a Valves with elongate central or terminal nodules 25 



24b Valves without elongate central or terminal nodules 26 



25a Central nodule drawn out to at least half the length 



of the valve Amphipleura (p. 62 ) 



25b Central nodule drawn out less than half the length 



of the valve Frustulia (p. 62 ) 



