Pyrenula chlorospila (Nyl.) Arnold

    Thallus crustose, immersed, sometimes glossy, olivaceous to greenish-ochre when moist, pale brown to fawn when dry, minutely dotted with pseudocyphella,  50-100 µm, visible as small white dots, prothallus paler, almost whitish in case of isolated colony, blackish amongst other mosaic-forming colonies. Perithecia numerous, sometimes arranged in concentric circles, almost hemispherical, markedly prominent, 0.2-0.4 mm diam., black. Ascospores ellipsoid, 3-septate, 1 angular locule, more or less diamond-shaped per cell, 28-32 x 11-13 µm. Photobiont: Trentepohlia. C-, KC-, K+ yellow, P+/- yellowish, UV +/- whitish. Common on smooth bark of trees, notably ash trees  (Fraxinus sp.). In mosaics, this species is often associated to Pyrenula macrospora. These two species may then be distinguished by the size of their perithecia. Pyrenula macrospora has much larger perithecia and in ascospores the locules have more curved angles; see also Pyrenula nitida that is K+ red-orange.