Perithecia (s. perithecium) are spore-containing structures of a lichen. They are usually immersed in the thallus surface, and appear as black dots on the surface of the lichen. Perithecia differ from other spore-containing structures by being almost entirely enclosed except for a small opening through which the spores exit–the ostiole.
Perithecia on Dermatocarpon reticulatum from Mount Elinor, Olympic Peninsula, WA. (A. Fischer--2/2011)