when the aperture becomes slit-like in the Cavoliniidae and the shell develops a bivalved appearance, tubercles at the inner side of ventral and dorsal 'valve' link the two and divide the aperture slit into three sections, with one pair of closing mechanism, or into five sections when a secondary lock system is present (see also: closing fold).

Alternative forms for closing mechanism : closing mechanisms, lock system, lock systems, primary lock system, primary lock systems, secondary lock system, secondary lock systems.