Ascomycota >Lecanoromycetes > Teloschistales > Caliciaceae.

Buellia has about 450 species of crustose lichens commonly known as disc or button lichens and they grow on wood,
    bark, rock but rarely on soil.

The thallus is crustose, rough or smooth, sometimes cracked (areolate) and may be somewhat squamulose or lobate.
It may be whitish, grey, brown or yellowish.
The edge may be diffuse or have a dark border of prothallus.

Apotheca (lecideine type) are usually common but may be absent where soredia are present.
The disc is initially flat then becomes convex and usually black but some are dark brown.
The hypothecium is usually a deep reddish brown but sometimes pale brown.
Outside this, the proper and thalline exciple layers may be absent or prominent then reduced with age.

Numerous other genera can have a similar appearance such as Heterodermia, Physcia and Rhizocarpon.

Buellia aethalea.
A crustose lichen with a cracked, light to dark grey or grey-brown surface.
There is a prominent dark prothallus seen at the edge and under the cracks.
The prothallus may be in the form of branched, radiating filaments.
The black lecideine apotheca usually have a thin margin.
A common lichen on rocks.

Rhizocarpon reductum.
A crust lichen with thalli up to 2 cm across which may coalesce.
The grey to brown upper surface is cracked.
The marginal prothallus is usually prominent.
The apothecia are black, immersed in the stroma or sessile and flat to convex.