Petractis clausa (Hoffm.) Kremp.

    Thallus endolithic, sometimes hardly visible, dull whitish, pale yellow-ochraceous. Apothecia more or less immersed in the substratum (and sometimes leaving a minute depression when falling down), 0.5-1 mm diam., disc pink-orange, pale orange, flat or concave, at first entirely covered with exciple that opens like mechanical claws with 4-8 radiating cracks star-shaped, determining an irregular exciple more or less persistent. Ascospores narrowly ellipsoid, 3-septate, 18-30 x 7-10 µm, surrounded by a well-developed gelatinous perispore that under microscope and after rehydration is crew-cut like or bottle-brush-shaped. Photobiont: cyanobacteria: Scytonema. Chemical spot tests negative. Rare, on vertical walls of hard calcareous rocks. N.B. Petractis luetkemuelleri is a very similar species but with smaller apothecia and a different photobiont: Trentepohlia.