Thallus superficial, well-developed, generally 250-800 µm thick (but up to 4-5 mm thick). Secondary areoles subdivided by dark lines into few to numerous (up to 30) small units. Secondary areoles with darker, roundish-angular margins, up to (0.5-)1-5 × 3-8(-15) mm, attached to the substratum by short black stipe-like holdfasts. Algal layer 60-100 µm thick, unpigmented, above a thin black basal layer.
Anamorph: not observed.
Teleomorph: Perithecia entirely immersed in the thallus. Involucrellum absent. Exciple colourless to pale brown, 120-250 µm diam. Ascospores simple in var. trachyticum, 1-septate in the var. subtrachyticum, 13-18 × 5-7 µm.
Compared with look-alike species the secondary areoles are often subdivided into up to 30 small units, and it does not begin as parasitic on other lichens. Separated from P. fuscellum by the absence of dark carbonaceous pigment in the lower part of the algal unit (not to be confused with the presence of a black basal layer which is present in both species). The var. subtrachyticum has 15% to 80% of 1-septate ascospores.
P. canellum is parasitic on Aspicilia calcarea. P. fuscellum has secondary areoles subdivided into fewer small units and has carbonaceous pigment in lower part of algal units. Verrucaria polysticta has perithecia arising between photosynthetic units.
In GB&I known currently (December 2017) from a single collection on concrete in a mainly brick-lined gutter at base of south wall of a church in VC 15, East Kent.
On Continental Europe recorded from a variety of calcareous and siliceous rocks, including bricks and mortar. In GB&I known currently (December 2017) from a single collection on concrete in a mainly brick-lined gutter at base of south wall of a church in VC 15, East Kent.