Placynthium (Ach.) Gray
Type : Placynthium nigrum (Huds.) Gray [=Lichen niger Huds.]
Descriptions : Flora (1985: 409). See also Schultz (2002b: 397).
Species of Placynthium are saxicolous, growing in ±moist places such as drainage cracks on rock faces, and the edges of lakes or rivers. Some species are found on dry limestone. Twenty-five species are described worldwide (Kirk et al. 2001), and are included in the family Placynthiaceae (Eriksson et al. 2004; Pennycook & Galloway 2004; Eriksson 2005). North American species were monographed by Henssen (1963c) and this account is still a useful treatment of species taxonomy in the genus. Sonoran Desert taxa are discussed by Schultz (2002b). Species are distributed mainly in the Northern Hemisphere and in the Southern Hemisphere the genus appears to be rare. Two species are known from New Zealand, one occurring widely on limestone outcrops and on man-made calcareous substrata (e.g. concrete), the other being a genuinely rare, bipolar species restricted to seepage cracks on schist outcrops in alpine grasslands of Central Otago (Galloway 2003b).
