208 



are present. The crustaceous appearance is frequently emphasized 

 by the presence of numerous soredia or isidioid outgrowths from the 

 upper surface. 



The upper cortical layer is well developed, the cells rather large, 

 irregular in form and position. The algal layer is also well developed, 

 the algae being in all cases colonies of Poly coccus functiformis of a 

 bright blue-green color and enclosed by a common gelatinous cover- 

 ing. The medullary layer consists of rather thick rigid branching 

 hyphae, quite frequently becoming somewhat cortical throughout. 

 The lower cortical layer is thinner than the upper and bears numer- 

 ous long simple usually black rhizoids. The upper surface of the 

 thallus is usually a reddish brown ; the lower surface is lighter in 

 color. 



The apothecia are rather small, disk-like, sessile upon the thallus, 

 the disk flattened with the thalloid exciple slightly raised. They are 

 numerous in the lower forms, few or wanting in the higher species 

 (P. lefidota) . The epithecium is reddish brown ; thecium and hy- 

 pothecium colorless, though the latter may be more or less tinged 

 with brown ; the spore-sacs are cylindrical with eight colorless simple 

 spores with pointed ends ; the exosporium is gelatinous and usually 

 of an irregular outline. 



I have excluded P. lanuginosa from the genus (see Pseudo- 

 lichenes) ; also P. molybdea, which does not possess a single char- 

 acter in common with the Pannarias proper. There are, no doubt, 

 other so-called Pannarias which should also be excluded. 



As here limited, the genus seems to be southern, extending, how- 

 ever, well into the north temperate zone, even into the arctic ; the 

 species occur upon moss, soil, rocks and trees. P. brunnea and a 

 few other species are frequently found spreading over Polychidium 

 muscicolum when this lichen occurs on moss. Both lichens seem to 

 thrive well, which makes it probable that we have here a form of 

 mutualism. 



PLATE 71. 



Pannaria lurida (Mont.). 



1. Natural size; a, apothecia. 



2. Section of apothecium. 



3. Section of thallus. 



4. Paraphysis and spore-sac. 



