Arthopyrenia cinereopruinosa (Schaer.) A. Massal.

     Thallus very thin and barely visible, slender (and may "peel"), smooth, whitish to pale greyish somewhat silvery and pearly. Perithecia black, 0.2-0.3 mm in diameter, possibly covered at first by fragments of thallus and leaving a "wheel" when they fall. Asci subcylindrical to clavate with 8 spores, 55-77 x 12-19 µm according to literature; 50-70 x 12-18 µm according to our measurements. Ascospores hyaline, elliptical to clavate with rounded ends and a perispore 1-2 µm thick, 1-septate, narrowed at the septum and also at the center of each cell giving them a 3-septate appearance at maturity, 15- 22 x 5-8 µm according to literature; 15-20 x 5-7 µm according to our measurements. Pseudoparaphyses thin, divided. Pycnidia sometimes abundant, black, 50-100 µm in diameter. No significant color reactions. Photobiont: absent (species considered non-lichenized, but can be found frequently in, or on, the thallus of Trentepohlia). Fairly rare species growing on the smooth bark of deciduous trees (here on Populus alba).

N.B. Beware of Mycoporum antecellens, quite similar, but with much larger spores and Arthopyrenia analepta whose spores are not shrunk to cells.

N.B. Watch out for Arthopyrenia punctiformis = Naeotrocymbe punctiformis practically without thallus, without pycnidia and with non-"strangled" spores and also with  Naevia (Arthonia)  punctiformis also practically without thallus, without pycnidia and with spores up to 5-septate.