Fastigium

A fastigium is an extension of the mantle margin at a valve apex of some species in genus Tursiocola. The fastigium extends from the valve margin towards the valve face. When observed in valve view, this extension of the mantle margin resembles a corner of a picture frame, a gable, a cornice, or eaves around a house. Fastigium is Latin for gable. The plural is fastigia.

The fastigium is only discernible in SEM.

fastigium-valve
Image Credit: Tom Frankovich
Valve view of Tursiocola varicopulifera with an arrow marking a fastigium (SEM). Scale bar = 1 µm.
fastigium-girdle
Image Credit: Tom Frankovich
Girdle view of a frustule of Tursiocola with arrows marking fastigia and one of the valve mantles (SEM). Scale bar = 2 µm.