10 



7a Punctae in multiseriate rows near valve margin, each row 

 gradually or abruptly becoming uniseriate toward the 



center; marginal spines always present 



Stephanodiscus (p. 28) 



7b Punctae in uniseriate rows throughout, or with geometric 

 facets Coscinodiscus (p. 31 ) 



8a True raphe present on at least one valve; raphe may be 

 very short or rudimentary, or may be concealed in a 

 keel or wing 15 



8b True raphe absent, pseudoraphe present on both valves... 9 



9a Frustules with thick longitudinal septae running parallel 

 to the valve faces Tabellaria (p. 32) 



9b Frustules without septae 10 



10a Valves with thickened internal transverse ribs (costae), 



most of which extend completely across the valve face... 11 

 10b Valves without thickened internal transverse ribs. 12 



11a Valves symmetrical about the transapical plane 



Diatoma (p. 33) 



lib Valves asymmetrical about the transapical plane 



Meridion (p. 36) 



12a Frustules with bulbous ends, slightly concave in girdle 



view, typically forming stellate colonies 



Asterionella (p. 37) 



12b Frustules without bulbous ends, typically not forming 



stellate colonies 13 



