Longitudinal rib

A solid silica structure that runs along each side of the raphe in genera such as Frustulia.

Longitudinal Rib
Image Credit: Carrie Graef
In Frustulia, the raphe lies between the two branches of the longitudinal rib.
Longitudinal Rib2
Image Credit: Sarah Spaulding
Species in Playaensis possess a longitudinal rib on either side of the raphe.