Annulus (Geissleria)

In the genus Geissleria, a circular or semi-circular hyaline area on the valve face which interrupts two or more distal striae. An annulus may be distinctive or barely discernible, depending on species. Plural form is annuli.

In Krammer et al. (2000) the definition is in the key for the Navicula group Annulatae. The species in this group have been elevated to the genus Geissleria.

Image Credit: Gene Stoermer
The annuli in this Geissleria are quite distinctive.