o 



194 LECANO-LECLDEEl [rHIZOCARPOX 



Appin, Argyll; Ben-y-Gloe, Ben Lawers aad Craig Tulloch, Blair 

 Athole, Perthshire ; Canlochan, Forfarshire ; Craig Gnie and Mon-one, 

 Braemar, Aberdeenshire ; Portmamock, near Dublin. 



9. Rh. petraeum Massal. Bic. 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 black, 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 CoUectan. Botan. iii. 

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

 Soc. n. p. 284 (1794) ; Engl. Bot. t. 246 ; With. Arr. ed. 3, iv. 

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

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

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

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

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

 petrstum var. concenirieum Mudd Man. p. 220 (1861). 



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



The specific name petraitm has been given by later British authors 

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

 tion and figure of Lichen petrctua undoubtedly represent this species 

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

 dence of the more characteristic concent i-icits of JJavies. 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, by the peculiar lines formed by the contiguous apothecia. 

 Leighton's f. iypica (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 rarely schistose and 

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

 B. M. Wadebridge, Cornwall; near Beeding and Sullington Heath, 

 Sussex ; UUacombe, Dartmoor, Devon ; Leith Hill, Surrey ; Wickwar, 

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

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

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

 sea ; Bilsdale, Cleveland, Yorkshire ; Pentland Hill s, near Edinburgh ; 

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

 Perthshire; Eillamey, Kerry. 



Var. excentricum A. L. Sm. (non Boist. ISTouv. El. Lich. pt. 2, 

 p. 240 (1902). — Thallus whitish, effuse, less developed than in 

 the species, sometimes almost evanescent. Apothecia numerous. 



