276 Rev. W. A. Leighton on the British Graphideae. 



horizontal patches, bounded by a pale blackish watery line, more 

 or less distinct and conspicuous, not unfrequently almost entirely 

 obliterated. Lirella very numerous, either crowded or more 

 distant, disposed in a horizontal direction parallel to each other, 

 generally rather long and narrow and of the same width through- 

 out, though with others of various lengths interspersed, acu- 

 minate and sharply pointed at each extremity, though sometimes 

 obtuse, simple and nearly straight, only slightly curved, some- 

 times by confluence branched, prominent and emergent or burst- 

 ing through the thallus, which is uplifted and forms membranous 

 slightly thickened thallodal margins. Proper margins stout, 

 thickened, rounded, prominent and uniform, enclosing a canali- 

 culate disk, generally narrow and almost rimseform, frequently 

 more expanded, either entirely destitute of or with only a faint 

 pruina. 



Distinguished from all other varieties of serpentina by the 

 horizontal arrangement of the elongated, narrow, simple, parallel 

 lirellae; and from scripta and pulverulenta by the sporidia.. 



Chevalher (Hist. Graphid. p. 16) says that his Ope^. Cerasi is 

 not that of Acharius, which he regards as a variety of scripta. 

 The lirellse represented in his tab. 1. fig. 3 & 4 are certainly 

 blunter and not acuminate at the extremities, and also appear 

 interwoven or lying one over the other, not preserving the 

 parallelism so peculiar to our plant. Possibly Chevallier's plant 

 may be a state of Ope^. atra. 



Plate VI. fig. 20. a. Vertical section of thallus and lirella; b, sporidium. 



4. Gr aphis diffracta. Turn. MS. Thallus thickish, tartareous, 

 pulverulent, cracked, rugulose; lirellse immersed, slender yet 

 clumsy, of the same width throughout, extremities obtuse; pro- 

 per margin narrow, elevated, wavy ; thallodal margin very nar- 

 row, almost obliterated ; disk canaliculate, naked ; sporidia eight, 

 in asci, linear, margined, rounded at the ends, containing 8-10 

 transversely oval margined yellow spores. 



The thick, tartareous, cracked thallus and the peculiar form 

 and habit of the lirellse, traceable through a distinct series, appear 

 to justify the separation of this from G. serpentina ^ in which the 

 thallus partakes generally of a more membranous structure, but 

 whether it will be permanently regarded as a distinct species 

 may perhaps be doubtful, among plants so protean and trickey 

 in their variations, particularly as the sporidia are similar. The 

 varieties spathea and eutypa of G. serpentina alone approach it in 

 their sub tartareous thallus. 



Plate VI, fig. 21. a, Vertical section of thallus and lirella; b, sporidia. 



