COLLKMACEI. 



[PTERTGITJM. 



Fig. 6. 



Ptcrygium centrifugum Nyl. a. Longitudinal section of thallus, X 275. 

 b. Transverse section of the same. c. Vertical section of a spermogone, 

 X42. d. Jointed sterigmata. 



This genus is analogous to Pannularia among the Lichenacei, from 

 which, however, it is distinguished by the absence of a hypothallus. In 

 this country few species have occurred, but probably others will be 

 detected. 



1. P. centrifugum Nyl. Bull. Soc. Bot. i. (1854) p. 328 ; Syn. 

 i. p. 92, t. ii. ff. 11-15. Thallus small, laciniato-radiate, centrifu- 

 gal, dark- or chestnut-brown, often blackish in the centre ; lacinia? 

 divaricato-multifid, plane or somewhat convex, narrow, crowded, 

 very thinly longitudinally rugulose. Apothecia unknown. 



Externally this looks like a small Physcia near to dark vars. of Ph. 

 obscura ; but its anatomical structure is essentially different. The thallus 

 is only about \ inch in diameter (even smaller in the only British spe- 

 cimen seen), and is more or less centrifugal. The gonimia are pale- 

 greenish, often moniliform towards the upper surface. No apothecia 

 have vet been detected, but the spermogones (not present in our spe- 

 cimen) are known ; they are tuberculose, usually blackish above, with 

 multi-articulate sterigmata ; spermatia straight, slightly acute at the 

 apices, 0,005-0,006 mm. long, 0,001 mm. thick. 



Hab. On calcareous rocks in a subalpine hilly district. Distr. Extremely 

 local and scarce, having been found only very sparingly in S.W. England. 

 B. M. : Cheddar Cliffs, Somersetshire. 



2. P. pannariellum Nyl. Sallsk. pro F. ct Fl. Not. iv. (1859) 

 p. 236 ; Flora, 1809, p. 41. Thallus moderate, laciniafco-radiato 



