Hypotrachyna horrescens

Thallus tightly appressed, 2–7 cm diam., ± irregular or forming small rosettes; lobes crowded, to 3 mm broad with subtruncate apices; margins frequently irregularly incised, often becoming lobulate, with abundant, mostly unbranched cilia; upper surface grey-white, matt or partly shiny, isidiate; isidia laminal, dense and ± obscuring the thallus, cylindrical or flattened when well-developed and becoming ± lobulate-coralloid, often dark-tipped with cilia growing from base or tips of isidia; lower surface black, ± brown towards the apices; rhizines unbranched or rarely squarrose or dichotomously branched. Apothecia very rare, sessile, 2–4 mm diam.; disc concave, split at maturity, dark brown to red-brown; thalline margin irregularly incised and ciliate-isidiate. Ascospores broadly ellipsoidal, 12–15 × 9–11 μm. Pycnidia not reported for British specimens. Cortex K+ yellow; medulla C–, K–, KC+ rose-red, Pd–, UV– (atranorin, sometimes also gyrophoric acid and a complex of medullary substances).

Hypotrachyna minarum lacks the laminal cilia associated with isidia, which are never flattened, and has a C+ pink medulla (gyrophoric acid major). Parmotrema crinitum also has ciliate isidia but is distinguished by the larger thallus, the broad (around 5 mm wide) marginal zone without rhizines on the lower surface and by the K+ yellow medulla (stictic acid).