88 Rev. W. A. Leighton on the British Graphidese. 



* Saxicola. 



1. O. tesserata, DC, Thallus tessellato-areolate ; lirellse 

 slightly prominent, short, obtuse, dark brown ; sporidia eight in 

 asci, minute, oblong, pale, hyaline. 



Opegrapha tesserata, DC. Flor. FraTi9. 2.313(1805); Chevallier, Hist. 



des Graphidees, 51. t. 11. f . 1 ; Bon-. E. Bot. Suppl. 2632. fig. 2 



(good) ; Hook. Br. Fl. 2. 146. 



. petrcea, Ach. Syn. 72 (1814); Fries, L. Ref.362. 



saxatilis, /3. tesserata, Scha;r. Enum. 159 (1851). 



Holwick Scar on the Tees, Yorkshire ! Mr. W. Robertson in 

 herb. Borrer. Valley of Rocks, Linton, North Devon, Mr. Borrer. 



Nothing need be added to the characteristic figures and de- 

 scriptions both in ' E. Bot. Suppl.^ and Chevallier^s work, but to 

 remark that the edges of the areolse within the interstices are of 

 a brownish-black colour, as if there were a dark substratum. By 

 reason of the firm and persistent nature of the pale gelatinous 

 contents of the lirellse, the examination under the microscope 

 was scarcely so satisfactory as could have been desired. It ap- 

 peared to consist of erect oblong asci, in each of which were 

 eight rather minute oblong pale sporidia, apparently without 

 septum. 



The thallus and sporidia distinguish this from 0. rupestris and 

 O. saxigena ; the thallus, sporidia, and lirellse from O. cerebrina, 

 O. saxatilis, and 0. Chevallieri. 



Plate V. fig. 1. a, Vertical section of thallus and lirella; b, sporidia; 

 c, ascus. 



2. O. cerebrina, DC. Thallus continuous, chalky-white; li- 

 rellse prominent, lumpy, rounded or squarish, full-black ; spo- 

 ridia in asci eight, linear-oblong, margined, uniseptate, bluish- 

 slate or smoky colour, surrounded with a broad white margin. 



Opegrapha cerebrina, DC. Flor. Fran^. 2. 312 (1805) ; Chev. Hist, des 

 Graphidees, 6?. t. 12. f. 4 ; Borr. E. Bot. Suppl. 2632. fig. 1 ; Fries, 

 L. Ref. 363 ; Hook. Br. Fi. 2. 146. 



Lecidea plocina, Ach. L. Univ. 155 (1810); Syn. 16. 



cerebrina, Scha;r. Spicil. 136. 196 (1823-1836) ; Enum. 159. 



North of England ! Rev. J. Harrimann in herb. Borrer. 



The general aspect, colour and structure of the chalky con- 

 tinuous thallus distinguish this at first sight from Oj). tesserata. 

 The more lumpy, rounded or squarish form of the scattered, less 

 frequently grouped lirellse, which are more prominent and less 

 immersed in the thallus, their full-black colour, their thinner, 

 sharper and less swollen inflexed margins, and the singular spo- 

 ridia are the marked characteristic distinctions from that species. 

 Each sporidium seems enveloped in a hyaline membranous sac, 

 which appears as a white margin surrounding it, the proper mar- 



