Pyxis: A circumscissile capsule, the top coming off as a lid.*
Above is an image of a ripe Portulaca pyxis. Circumscissile means that the capsule splits horizontally in a line around the middle when it’s ripe. You can see the ‘lid’ of this one coming off, releasing the shiny black seeds inside.
*Harris, J.G., Harris, M.W. (2001). Plant identification terminology: An illustrated glossary (2nd ed.). Spring Lake: Spring Lake Publishing.