58 ANGIOCARPOUS LICHENS. 



In this species it would seem that the carbonaceous 

 dimidisite j)erit/ieciu?n is larger or smaller in extent ac- 

 cording to the size and age of the apothecimn, either 

 reduced to an almost speck when closely surrounded with 

 the thallus, in which it is wholly immersed, and then not 

 unaptly resembles the apothecium of a Sagedia, or of 

 V. ejjigaa, or HooTcerl ; or of a larger size when the apo- 

 thecium is found filling minute open cavities in the stone, 

 when the perithecium is thick and flattened, contracted 

 so as to fit the cavity. The inner tunic also differs in 

 thickness, being either very thin, or very nearly equalling 

 the thickness of the perithecimn, from which it is then 

 rather difficult to distinguish it. In all cases, however, 

 it is tough and membranous, and so capable of being 

 distinguished from the hard rigid carbonaceous perithe- 

 cium. The apof/iecia are sunk in small cavities in the 

 stone, and vary in size, being either large and scattered, 

 or minute and densely crowded. These forms may be 

 traced in gradual transition on the same specimen ; when 

 on separate specimens, as is frequently the case, their 

 general appearance is very dissimilar, and, at first sight, 

 they might be supposed to be distinct plants. In an ad- 

 vanced state, when the cavity of the stone becomes 

 enlarged, and the apothecium loosened, it would seem 

 that the carbonaceous dimidiate perithecium becomes 

 friable, and falls partially or entirely away, leaving the 

 thick tough inner tunic remaining, concave at the top, 

 surrounded or margined with the remains of the perithe- 

 cium. In this condition the apothecium would be 

 pronounced decidedly entire. Not unfrequently the 

 perithecium and the upper portion of the tunic falls off", 

 leaving the lower portion of the now open tunic remaining, 

 which finally falls out of the cavity in the stone. The 

 nucleus is variable in colour, when dry darker or paler 

 brown, when wetted pale and hyaline. Sporidia in asci 8, 

 linear-oblong, more or less slightly tapered towards each 

 extremity, uniseptate, hyaline. The thallus varies in 

 colour from a chalky whiteness to a pale dirty yellow, 

 and a grey ashey white. 



