Placidium A.Massal.
Type : Placidium michelii A.Massal.
Description : Thallus squamose, attached to substratum with rhizohyphae, lacking an umbilicus and rhizines. Upper surface smooth. Medulla of spherical and/or filamentous hyphae. Photobiont green, protococcoid, cells to 15 μm diam. Lower cortex paraplectenchymatous. Ascomata perithecia, laminal, almost completely immersed in thallus, lacking an involucrellum. Exciple hyaline. Hamathecium of periphysoids, without paraphyses. Asci cylindrical, bitunicate, 8-spored, the ascospores uniseriately arranged. Ascospores simple, colourless. Conidiomata pycnidia, Dermatocarpon -type, laminal and marginal. Conidia oblong–ellipsoidal or cylindrical.
Placidium, included in the family Verrucariaceae (Eriksson et al. 2004; Pennycook & Galloway 2004; Eriksson 2005), is characterised by a squamose thallus attached to the substratum by rhizohyphae, a paraplectenchymatous upper cortex, perithecia with a colourless exciple, but lacking an involucrellum; and pycnidia of Dermatocarpon -type (Harada 1993a: 137 – as Dermatocarpella; Breuss 2002d). Placidium is distinguished from Catapyrenium by the hyaline exciple, cylindrical asci with uniseriate ascospores, and the smooth upper surface. Tweny-eight species are known worldwide (Breuss 1996, 2001c, 2002d), one of which occurs in New Zealand.
