Thallus yellowish-ashy, parasitic on other lichens, irregularly chinky-areolate, areolae flat, dull, epruinose, soft; hypothallus lacking. Apothecia dispersed or grouped, round, to 1 mm broad but usually less, adnate; margin thick, waxy, shining, paler than disk and more yellow; disk flat to slightly convex, dark yellowish-rusty red, epruinose, smooth; hypothecium hyaline, dense; epihymenium brownish, granular-inspersed; hymenium 65-70 µm, hyaline; paraphyses free, slender, sometimes constrictedly septate above, sometimes slightly thickened to 4-5 µm, branching; spores 13-16 x 8-10 µm, septum 2.5-3 µm.

This species grows parasitically over other crustose lichens, particularly Placynthium aspratile, especially on seepage surfaces. It is known from Lapland and Greenland, and is found in the North American Arctic.

Hansen, Poelt, & Sochting (1987) accept C. castellana over C. invadens, pending clarification of the possibility of whether or not they may represent modifications of one species, or really may be distinguished by habitus. I have left them together in a broader interpretation of the possible species limits under the earlier epithet.

Several other parasitic species of Caloplaca have been reported by Hansen, Poelt, & S0chting (1987) from Greenland and should be sought in the American Arctic. These include C. psoricida Hansen, Poelt, & S0chting on Psora rubiformis, characterized by rust-red to brown apothecia and narrowly ellipsoid spores 14-18 x 5.5-6.5 µm, septum 3-5 µm; C. insularis Poelt on Aspicilia with brownish orange apothecia, the medulla and exciple 1+ violet, and spores 9-13 x 6-9 µm, septum 2-3.5 µm; C. magni-filii Poelt on Lecidea nigroleprosa, also with medulla and exciple 1+ violet, apothecia rust-red to orange, spores 10-12 X 5-7.5 µm, septum 2.5-3.5 µm; and C. anchon-phoeniceon Poelt & Clauz. in Poelt on Aspicilia with purple to carmine-orange apothecia, medulla and exciple 1—, spores 11-15 x 7-8 µm, septum 3-5 µm. Yet two other parasitic species, C. cacuminum Poelt, only a facultative parasitic species on other crustose lichens, and C. epithallina, are treated separately in this book.