194 LKCANO-LK<;iDKKl [rHIZOCARPON 



Appin, Argyll ; ]ien-y-Gloc, }Um Lawcv.s and Craig Tulloch, Blair 

 Athole, Perthshire ; Canlochan, Forfarshire ; Craig Guie and Morrone, 

 Braeinar, Aberdeenshire ; Portmarnock, near Dublin. 



9. Rh. petrseum Massal. Kic. Lich. p. 102 (1852) (non 

 Koerb.). — Thallus white or greyish- white;, orbicular, determinate, 

 thin, wrinkled or almost smooth, subcontinuous or cracked- 

 areolate, sometimes thin and almost disappearing (K — , CaCl — , 

 I — ) ; hypothallus evanescent. Apothecia ])lack, small, usually 

 growing in concentric lines, appressed or subinnate, somewhat 

 concave or plane, marginate, the margin thick and often 

 white-pruinose; hypothecium blackish-brown; spores oblong, 

 muriform, colourless or slightly brownish, halonate, 0,025-44 mm. 

 long, 0,011-17 mm. thick; hymenial gelatine deep-blue with 

 iodine. — Lichen petrseus Wulfen in Jacquin Collectan. Botan. iii, 

 p. 116, t. 6, f . 4 (1789). L. conceniricus Davies in Trans. Linn, 

 ^oc. ii. p. 284 (1794); Engl. Bot. t. 246 ; With. Arr. ed. 3, iv. 

 p. 18. Lecidea pctrsea Ach. Meth. p. 37 (1803) ; S. F. Gray Nat. 

 Arr. i. p. 463 ; Hook, in Sm. Engl. Fl. p. 175 ; Tayl. in Mackay 

 Fl. Hib. ii. p. 117 pro parte; subsp. concentrica Nyl. Lich. 

 Scand. p. 234 (1861) ; Cromb. Lich. Brit. p. 87. L. concentrica 

 Leight. Lich. Fl. p. 349 (1871) ; ed. 3, p. 378. Bhizocarpon 

 pett'Aum var. concentricum Mudd Man. p. 220 (1861). 



Exsicc. Leight. n. 17 ; Johns, n. 355. 



The specific name petrwuin has been given by later British authors 

 to forms now included under Rli. confervoides, but Wulfen's descrip- 

 tion and figure of Lichen jyeiTO-us undoubtedly represent this species 

 with its concentrically arranged apothecia, and his name takes prece- 

 dence of the more characteristic concentricus of Davies. Sometimes 

 it is regarded as only a variety of the preceding species, but is easily 

 recognized and differentiated, even when the thallus is almost evan- 

 escent, b^^ the peculiar lines formed by the contiguous apothecia. 

 Leighton's f. typica (Lich. Fl. ed. 3, p. 378) is a condition in which 

 the thallus is well developed and almost continuous ; in f. impressula 

 Leight. and f. coarctata Leight. {I. c. p. 379) the apothecia are more 

 concave and at times circumscissed ; in the latter the thallus is also 

 diffuse or scattered. The spermogones are not uncommon, the sper- 

 matia rod-shaped, 0,006 mm. long, 0,0006 mm. thick. 



Hab. On rocks, chiefly calcareous, more rarelj schistose and 

 arenaceous. — Dlstr. Frequent in maritime and upland regions. — 

 B. M. Wadebridge, Cornv/all ; near Beeding and Sullington Heath, 

 Sussex ; Ullacombe, Dartmoor, Devon ; Leitli Hill, Surrey ; Wickwar, 

 Gloucestershire ; Malvern Hills, Worcestershire ; near Ledbury, Here- 

 fordshire ; Oswestry, Shropshire ; Cader Idris and Dolgelly, Merioneth ; 

 Capel Curig, Carnarvonshire ; Llangollen, Denbighshire; I. of Angle- 

 sea ; Bilsdale, Cleveland, Yorkshire ; Pentland Hills, near Edinburgh ; 

 near Balmerino, Fife ; Baldoran, Forfarshire ; Glen Lochay, Killin, 

 Perthshire ; Killarney, Kerry. 



Var. excentrieum A. L. Sm. (non Boist. Nouv. Fl. Lich. pt. 2, 

 p. 240 (190^). — Thallus whitish, effuse, less developed than in 

 the species, sometimes almost evanescent. Apothecia numerous, 



