Parmelina pastillifera  

Thallus 4-8 (-15) cm diam., closely appressed, rosette-forming; lobes 3-7 (-10) mm wide, sublinear, contiguous or slightly overlapping, somewhat undulating centrally; margins broadly rounded but irregularly indented, short-ciliate in axils; upper surface mineral grey with blue tinge, faintly maculate, often pruinose towards the apices, isidiate. Isidia blue-black, scattered or densely covering central lobes, laminal, short and initially flat-topped, tips developing a raised margin (button-like) and becoming knob-like and contorted, at times becoming confluent. Lower surface black, brown towards the lobe margins; rhizines continuing up to lobe margins, simple.
Anamorph: not seen in British material.
Teleomorph: ascomata apothecia, not seen in British material but reportedly similar to those of Parmelina carporrhizans.
Chemistry: cortex K+ yellow. Medulla C+ carmine-red, K–, KC+ red, Pd–, UV– (atranorin, lecanoric acid).