Blastenia crenularia (With.) Arup, Søchting & Fröden = Caloplaca crenularia (With.) J. R. Loudon

    Thallus crustose, rough, rather thick, cracked-areolate, up to 10 cm max. length, whitish to pale gray but often appearing dark gray or slate by its prothallus. Apothecia very numerous, up to 1 mm diam., rounded but occasionally slightly flexuose, disc flat then convex to (seldom) bulging, bright rusty, reddish, dark brown-red, almost blackish, seldom red-orange, sometimes with green or black spots, but these spots never cover entirely the disc especially when wet, proper exciple thin, typically, minutely and transversally pleated or crenulate, often winding, smooth and glossy when young, then insconspicous and concolourous to disc. Ascospores ellipsoid, polarilocular, septum 5 µm wide, to 1/2 of the length of the ascospore, the two poles linked by a thin conspicuous isthmus-like junction, 12-14 x 6-8 µm according to literature, 12-17 x 7-10 according to our measures. Thallus K- ; apothecia K+ purple. Photobiont: Trebouxia. Not rare on dry and well lit siliceous or weakly alkaline rocks on the seashore, but occasionally also inland, on old walls. 

    This species is considered here sensu lato since it appears highly variable and many taxa coexist. In harsh conditions, specimen are limited to a thallus covered by minute brown-red granules or distorted apothecia (see Caloplaca crenularia forma and Caloplaca crenularia var. contigua). In case of dry weather, this species is hardly distinguishable from Caloplaca ceracea which can be found close (see picture), but  usually prefers less sunny and more moist conditions, but apothecia discs of the latter are more dull yellow-orange and some of them possibly entirely green if not too dry, with margins paler and not concolourous and discs not bulged. However, many intermediate specimen exist and some are rather difficult to determine without microscopic observations.

N.B. Closely related to the bark-inhabiting Caloplaca ferruginea, as if it was its double.

_ See also Caloplaca arenaria of which the apothecia are smaller, less red and more orange, the thallus is thin and hardly visible and ascospores have a narrower equatorial thickness (septum) and Caloplaca fuscoatroides with a thick, dark gray to blackish thallus with more orange apothecia and large black protruding pycnidia which is strictly limited to rocks and large siliceous stones of the xeric supralittoral zone.