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Pyrenulales (subdivision Ascomycotina) An order of saprotrophic and lichenized fungi in which the thallus is crustose and the ascocarps are perithecioid (see PERITHECIUM). The asci are bitunicate. Spores are multiseptate and colourless or brown. There are several genera, including Pyrenula. P. nitida is common on trees in moist western districts of Britain. The thallus is smooth and green to yellowish-grey, typically surrounded and intersected by black lines. The perithecia appear as black dots up to 1 mm across.