Description of the type specimen. Thallus resembling Lecidea paupercula in gross morphology; crustose, areolated. Cortex K-, P-, C-. Medulla K+ reddish (norstictic acid), I-. Areoles dispersed on a black prothallus, roundish (irregular) in outline, rather flat, shining (due to a well developed epinecrotic layer), dark brown when gray (sic), light brown when moist, thin (up to 0.2 mm tall). Apothecia, some 10–30 per cm2, black, not pruinose, flat, up to 2.0 mm in diam., strongly adnate to substrate, without constricted base and not overtopping areoles, with small, prominent, shining margin and flat, dull black disc. Epihymenium 10–15 µm tall, bright green (with pigment Cinereorufa-green). Hymenium 40–50–70 µm tall, colorless, I+ blue. Subhymenium 20–25–35 µm tall, unpigmented. Hypothecium 70–120 µm tall, dark brown. Exciple unpigmented, except olive-black, 15–25 µm thick outer rim, densely inspersed by colorless crystalline masses and built by densely arranged, radiating, thick (3.5–5.5 µm) hyphae with apical cells up to 6 µm in diam. Paraphyses simple, rarely branched or anastomosing, 1.2–1.7 µm thick, with apical cells up to 4 µm in diam. Asci of Lecidea-type. Ascospores [n=60] bluntly ellipsoid, 7.5–9.0–12 X 3.0–4.0–5.0 µm. Pycnidia immersed, tiny. Pycnospores bacilliform, 8–12 X ca 0.7 µm.
Lecidea lygotropa is a member of the Lecidea atrobrunnea group. Its spore size agrees well with those of Lecidea atrobrunnea, while its general appearance (especially its large, flat, strongly adnate and marginally not constricted apothecia) point to Lecidea paupercula (that has considerably larger ascospores). Both taxa usually show a deep I+ violet reaction in the medulla. However, specimens lacking the positive amyloid-reaction occur, although rarely, in both taxa. For Lecidea paupercula and the closely related L. praenubila (with colorless or very pale hypothecium), the stictic acid syndrome is characteristic, although specimens not producing lichen substances often occur. A norstictic acid race is still unknown in Lecidea paupercula. However, a norstictic acid-chemotype occurs—among various other chemotypes—in Lecidea atrobrunnea s. l. (Leuckert & Hertel 2003). For the moment we accept Lecidea lygotropa as a separate species. It was erroneously reported for Tajikistan by Andreev and Kudratov (1989).