Similar in appearance to Arthonia ilicina, but this has ascospores with a muriform "tail" and are distinctly warted when brown. . Also similar to Arthothelium macounii, but this has K+ purplish and magenta pigments in teh epithecium and hypothecium.
On smooth (Corylus, Ilex and Sorbus) bark or roughish bark (Salix) in old woodlands.
A possible British endemic, and the old lichen "flora" mentions only the W. & C. Scottish Highlands. There appear to be no records from Wales so it is something of a mystery why it is on the Welsh S42 list. (But Steve Chambers thinks it may be in Alan Pentecost's Gwynedd Lichen Flora). Also, N. Wales is given in the new lichen flora.