34 COLLEMACEI. (PTERYGIUM. 
; 
Fig. 6. 
Pterygium eentrifugum Nyl.—a. Longitudinal section of thallus, x 275. 
b. Trance 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; laciniz 
divaricato-multifid, plane or somewhat convex, narrow, crowded, 
very thinly longitudinally rugulose. Apothecia unknown. 
Externally this looks like a small Physeia 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 yet been detected, but the spermogones (not present in our spe- 
canes) are known; they are tuberculose, usually blackish above, with 
multi-articulate sterigmata ; etme 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 8.W. England. 
—B.M.: Cheddar Cliffs, Somersetshire. 
2. P. pannariellum Nyl. Siillsk. pro F. et Fl. Not. iv. (1859) 
p- 236; Flora, 1869, p. 41.—Thallus moderate, laciniato-radiate 
