Caloplaca holocarpa (Ach.) A. E. Wade
Caloplaca lithophila H. Magn. Nomen conf. et inapproprié ?
Thallus crustose, sometimes slightly immersed, thin, areolate and cracked but most often inconspicuous or discontinuous, developed only around apothecia, greyish yellow to dull grey-orange, prothallus normally absent, soredia absent. Apothecia usually abundant, scattered to crowded, often contiguous, 0.3-0.7 mm diam., disc ± flat to convex when mature, yellow, orange-yellow to bright orange (mountains specimen), margin well delimited at first then inconspicuous or absent, concolorous. Asci cylindrical, 8-spored. Ascospores polaribilocular, ellipsoid to broadly ellipsoid, colourless, 10-13 x 5,5-7,5 µm, septum 3.6-5.4 μm wide, ratio of septum/spore length 0.31-0.45. Not rare, including in towns, on siliceous stones of dull and dusty old walls, on siliceous rocks, mortar, concrete, seldom on bark of deciduous trees, and in that case usually at the base.
We stricto sensu consider this species according to : ARUP Ulf. "The Caloplaca holocarpa group in the Nordic contries, except Iceland" The Lichenologist. 41 (2): 111-130 (2009). This publication is based on molecular analysis of the nrlTS DNA gene. Within this group, it seems appropriate to distinguish Caloplaca holocarpa from other species of the group notably those characterized by ascospores with wide septum:
Caloplaca oasis of which apothecia are brown-orange and grows on limestone and concrete near or on Verrucaria sp.
Caloplaca vitellinula (Nyl.) H. Olivier, which is sometimes extremely hard to distinguish from Caloplaca holocarpa. The main difference from Caloplaca holocarpa is the better developed thallus, areolate, cracked, grey-yellowish, and the normally scattered apothecia, yellow to orange, instead of the crowded ones in Caloplaca holocarpa. Grows on vertical walls of siliceous rocks or under overhangs.
Caloplaca pyracea (Ach.) Th. Fr. of which the thallus is well developed and apothecia bright orange, on bark of deciduous trees (Populus).