89 



Fig. l^l. Surirella striatula , (5), cells 80-260u long, 50-l60u 

 wide; longitudinal folds very wide, 0.6-1.2 in lOu; 

 small distinct protruberances on the outer edge 

 of the longitudinal folds. X 400. 



valve view 



Notes : 



42a Keels eccentric to the median axis, diagonally opposite jfrustules 

 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 occasion- 

 ally forming irregular ribbon-like chains or stellate colonies - 

 NITZSCHIA _ keel 



Diagrammatic cross-section of Nitzschia , (7). 



