Thallus giving a gray color to the stone and within it. Perithecia immersed in pits in rock, only the tips showing and leaving cavities in rock when they fall out; 0.25-0.3 mm broad, black; exciple entirely black and slightly widening toward ostiole; involucrellum lacking; gelatin 1+ red; spores hyaline to pale yellowish brown, not dark brown, muriform with many cells, 6-8-septate transversely, 1- or 2-septate longitudinally, 30-45 x 15-24 µm.

This species grows within limestone rocks and pebbles. It is not commonly collected; records are from Sweden and the Alps in Europe. In North America there are records only from Fort Churchill, Manitoba, and Prince Patrick Island, Northwest Territories.

This species is very similar to P. albida Arnold, which the Zahlbruckner Catalogus lists as occurring in North America, but has larger spores (they are less than 35 µm long in P. albida) and the spores remain hyaline in P. albida. It also appears very close to P. amota Arnold, which has similar habit and exciple, but has larger spores, 30-52 x 18-28 µm, and contains many more cells, being divided

8-10 times transversely, 4-6 times longitudinally. P. amota replaces P. obsoleta in the alpine habitats in Europe. If these prove to be the same species, P. amota is the older name (1869). Of the three names, P. albida is the oldest (1858). P. albida was reported from Newfoundland by Arnold (1899).