Arthonia calcarea (Turner ex Sm.) Ertz & Diederich s.l. = Opegrapha calcarea Turn ex Sm. s.l.

    Description sensu lato. Thallus highly variable according to substrata, thin, smooth or slightly rimose or cracked, sometimes partially endolithic, usually poorly delimited, whitish, greyish, pinkish, ochre-rusty, ochrish etc. Apothecia lirelliform, lirellae black, scattered or in clusters, in stacks, 0.5-2 x 0.2-0.3 mm, variable: from simple to forked or stellate. Ascii clavate along their length or cylindrical, hidden in a dextrinoid tissue when young then this tissue becomes amyloid, 80-90 x 15-20 µm according to our measurements. Ascopores  clavate with a larger side, 3-septate, 16-20 x 4-6 µm according to literature, 14-18 x 4.5-5.5 µm according to our measurements. C-, K-, KC-, Pd-. Theoratically, as suggested by its name, grows on calcareous rocks and mortar of old walls; however, practically, grows on all types of rocks provided that they are not too acid or too smooth. For instance, see Arthonia calcarea morph on painted mortar. N.B. If thallus is lacking, lirellae are bright and rock acid, see Opegrapha conferta.

Arthonia calcarea is a highly variable species (an aggregate or a collection of several morphotypes) from which various attempts to isolate several species have been done, sometimes considered by some authors as morphotypes since intermediate specimens with the canonical type may be found. Additional research is required to clarify their taxonomical rank. We present here pictures of this 'species' sensu lato and for information purpose only.

Arthonia calcarea forma (Opegrapha chevallieri) Leight, that grows on cement of maritime works near the seashore and notably piers and dikes. Distinguished by a thallus thick and white, breaing lirellae aggregated in small piles up to 5 mm diam. Nowadays considered as a morph of Arthonia calcarea but that could be its type in our prospection region along the sea.

Opegrapha conferta auct. Thallus lacking or thin and evanescent, grey, coming in fissures of neutral to acid coastal rocks. Lirellae variable, sometimes nodulous and in chains, up to 7 mm long, bright black. Species maintained in the Opegrapha group.

Opegrapha saxatilis auct. non DC. Thallus whitish to very pale brownish, more or less endolithic on hard limestone. Nowadays seems to be considered by some authors as a morph of Arthonia trifurcata.

Arthonia trifurcata with long and thin lirellae, placed in stars and more or less burried in the substratum, species limited to hard limestones.