Trapelia collaris Orange

     Thallus relatively thick, forming a coherent and well-defined crust, cracked and areolate, areoles 0.2-0.5 mm (sometimes longer towards the margin), convex, pale greyish, dirty whitish. Apothecia immerged in the thallus at first then sessile, small, 0.2-0.3 mm in diameter, sometimes, but rarely, with small remnants of the thallus at the margin, often with a pale brown "collar" of cartilaginous appearance, disc flat, reddish-brown, rapidly becoming dark brown or blackish (apothecia are sometimes aborted and misshapen and may show regeneration in the centre). Asci almost cylindrical, with (4) 8 spores, 85-105 x 15-21 µm according to our measurements. Ascospores elliptical, simple, hyaline, 16-22 x 9-12 µm according to the literature, 16-20 (23) x 9-11 µm according to our measurements. C+ red (sometimes fleeting), K-, KC-, P-. Species growing on moist siliceous rocks of scree, mine or quarry waste (particularly slate shale), rich in heavy metals and iron. Typically with Rhizocarpon oederi, Placopsis lambii. To be differentiated from Trapelia obtegens sorediate (but not always), with more convex areoles and larger apothecia.

N.B. In the field, watch out for malformed thalli of Placopsis lambii