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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.