Planes and axes of symmetry are employed in keying out some of the pennate 

 genera. They are shown in the figures below. 



Oblique view of a pennate frustule showing planes; apical plane 

 (along longitudinal or medial axis), a; transapical plane (along 

 the transverse axis), t. 



Oblique view of a pennate frustule showing axes : apical axis , 

 a; transapical axis, t; and the pervalvar axis, p. 



