A space or gap in the apical silica of the
between the apex and the subapical projections on the sides of the ring. May have a pyriform shape (have the shape of a pear).
Specific to the genus Mastogloia. Visible with SEM; usually not discernable with LM. From Latin for gap. Plural form is lacunae. An example species having a lacuna is Mastogloia calcarea.
Paddock, T.B.B. and Kemp, K.D. (1990). An illustrated survey of the morphological features of the diatom genus Mastogloia. Diatom Research 5: 73-103.