What is peristome teeth ?!

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Most mosses produce a capsule with a lid (the operculum) which falls off when the spores inside are mature and thus ready to be dispersed. The opening thus revealed is called the stoma (meaning "mouth") and is surrounded by one or two peristomes. Each peristome is a ring of triangular "teeth" formed from the remnants of specially thickened cell walls. There are usually 16 such teeth in a single peristome, separate from each other and able to both fold in to cover the stoma as well as fold back to open the stoma.