Thallus crustose, effused, endolithic, whitish or yellowish-white, sometimes with scattered blue-grey flecks. Photobiont Scytonema.
Anamorph: not seen in GB&I material.
Teleomorph: ascomata apothecia, 400-700 µm diam., usually paler than the thallus, flat to shallowly convex, urceolate, immersed, leaving pits in the rock when decayed, developing below a circular thalline cover which splits radially from a central pore as the structure expands. Thalline margin ± persistent, 10-15 μm wide, ± expanding to 30 μm in the uppermost part, initially covering the apothecia, with (3-) 4-6 (-8) distinct radiating fissures developing at maturity, disintegrating with age. Disc pinkish to pale orange or pale yellow-orange when moist, sometimes drying reddish-brown. True exciple pale, thin, inconspicuous, detached from thalline cover when dry, composed of conglutinated narrow hyphae. Hymenium 200-320 μm tall. Interascal tissue of slender, simple, thinly septate paraphyses 1-1.5 μm diam., scarcely swollen at the colourless apices; periphyses absent. Asci 150-250 × 12-18 μm, ± cylindrical, the wall thin, with a single functional layer, J+ blue; without a distinct apical dome, 8-spored. Ascospores (15-)17-23(-35) × 5-7 μm (excluding the perispore), mostly 3-septate, rarely submuriform, hyaline, fusiform, with a conspicuously fimbriate sheath.
Chemistry: lichen products not detected by TLC.
Asssessed by Woods & Coppins (2012) as of Least Concern. It is widespread where suitable habitat occurs, and locally frequent.
Recognized by the immersed apothecia with a ± raised thalline covering layer with fissures that create a star-like appearance; the young apothecia can be misinterpreted as perithecia.
There are several look-alike species. Ramonia calcicola has a very similar appearance, but its apothecia are smaller (<400 µm diam.) with the true exciple composed of angular cells (visible in water mounts). Gyalecta hypoleuca is an unrelated species that was once placed in Petractis and has an open disc surrounded by a thalline margin that is less well-developed than that of P. clausa but which may also be radially fissured. It is easily distinguished by its ascospores, which are usually 5- to 9-septate, and lack a perispore. Petractis nodispora is a recently recognized species from south Wales, with a pale pink endolithic thallus and a Trentepohlia photobiont, fairly conspicuous nodulose conidiomata and broadly open ascomata surrounded by a thalline exciple that is hardly fissured. Molecular data suggests that this species is distinct from Petractis at generic rank, but more data are needed to pinpoint its place on the evolutionary tree.
In limestone habitats from Devon in the south to Skye in the north and scattered throughout Ireland, with distributions focused on Dorset and the Pennine limestone pavements. BLS map here.
On highly calcareous rocks usually in ± damp situations, especially where partly shaded; often on loose limestone or chalk pebbles on the ground.