 The theca are divisible into left and right sides longitudinally by a sagittal suture; there is also a latitudinal girdle.Structurally complex, the genus is characterized by possession of extensive girdle and sulcal lists (wing-like extensions of the cell wall).Their theca have numerous pores which open flush to the surface of the plate on the outside but have a raised rim on the inside, their number being positively correlated with cell size.