[Radiolaria Phaeodaria] The main, cap-shaped to conical portion of the astropyle (Phaeodarian cell features, Phaeodarian cell features 8 inner shell).

[Pteropoda] Plate closing off the shell aperture when the animal is retracted; it consists of whorls around a nucleus or gyre (Peraclis in oral view). The operculum formula is L:G:H:W:D:E, the letters standing for greatest operculum diameter : greatest gyre diameter : greatest last whorl diameter : smallest operculum diameter : smallest gyre diameter : smallest last whorl diameter.

Alternative form for Operculum : [Pteropoda] operculum formula.