Rev. W. A. Leigh ton on the British Graphidese. 439 



Arthonia astroidea, Ach. in Schrad. N. Journ. Bot. 1. B. 3 st. p. 1/. t. 4. 



figs. 4 & 5 (1806) ; Johnston 1 Fl. Bern. 2. 101. 

 Arthonia radiata, oc. /3. y. b, Ach. L. Univ. 144 (1810). 



astroidea, u. df 13, Ach. Syn. 6 (1814). 



radiata, Cheval. Fl. Paris, 540 (1826). 



microscopicay y. asterograpta, Wallr. Crypt. Germ. 1. 322 (1831). 



vulgaris, var. astroidea et radiata, Schaer. Spicil. 8.246(1823-1836); 



Lich. Exsic. 16! 

 Opegrapha atra * macularis, Fries, L. Ref. 367 (1831); (in part) Hook. 



Br.Fl. 2.145 (18>3). 

 atra, X. radiata et jx. astroidea, Scheer. Enum. 154, 155 (1851). 



Sussex ! Mr. Borrer. Berwick-on-Tweed ! Dr. G. Johnston. 



Thallus thin, membranaceous, smooth, continuous, sometimes 

 slightly shining, frequently cracking and scaling off, either milk- 

 white, or pale yellow or cream-colour, or of a paler or darker 

 olive colour, slightly raised into a thin membranous spurious 

 thallodal border around the ardellse, which in a young state are 

 entirely covered by this as by a thin veil, which bursting makes 

 way for its emergence, circumscribed either entirely or partially 

 by a watery wavy border or line, which is either pale and very 

 indistinct, or distinct and of a darkish brown, forming irregular 

 transversely elongated patches of an oblong form of various size 

 and extent on the smooth bark of trees. Ardellce slightly pro- 

 minent above the surface of the thallus, very numerous, closely 

 approximate and even crowded, yet each distinct and separate, 

 of an irregular round or oblong or linear-oblong shape, congre- 

 gated many together and forming minute clusters, which by 

 confluence become pedato-stellate or radiate, and in a still later 

 stage of confluence become indistinctly stellate, or rather irre- 

 gular angular blackish-brown flattened spots. Disk blackish or 

 dark brown, plane and expanded, tumid and convex when moist- 

 ened, rugged or roughened on the surface, without any percep- 

 tible proper margin, except the pale very narrow thallodal mem- 

 branous margin. In a rubbed state the lamina proligera disap- 

 pears and the lateral receptacle becomes visible, giving the ap- 

 pearance at first sight of a margined disk. A vertical section 

 shows the pale lamina proligera supported on either side by a 

 dark brown perithecium which does not subtend the base, and 

 appears of very little greater consistence than the dark surface 

 of the disk. 



The so-called " varieties" seem rather states of the same plant 

 than varieties properly speaking. 



Plate VIII. fig. 32. a, Vertical section of thallus and ardella ; b, sporidia. 



4. Arthonia Swartziana, Ach. Thallus thin, cartilagineo- 

 membranaceous, scaly, cream-colour or ashy-gray, subdeter- 

 minate; ardellse subsessile, clustered into irregular angular or 



