205 



structure is, of course, radial, and, as in Omphalarta, the algae seem 

 to have undergone considerable change from the normal type. 



The apothecia are usually few in number, small, disk-like, ses- 

 sile upon the basal portion of the thallus, never terminal. The hy- 

 pothecium is colorless, well developed and typically cortical ; the 

 thecium is colorless, the paraphyses rather thick, simple, colorless ; 

 the epithecium is light brown ; the spores are comparatively large, 

 oblong, elliptical or spindle-shaped, sometimes slightly curved, color- 

 less, two-celled. 



It is evident that in its histology Polychidium is almost without a 

 parallel. The algae and fruticose structure exclude it from Lepto- 

 gium ; the structure and position of the apothecia exclude it from 

 Lichina and Omphalaria ; its resemblance to Ephcbe is only appar- 

 ent ; it is withal a difficult genus to classify, its present position being 

 only tentative. 



As far as known, P, nuiscicohim is rather northern in its dis- 

 tribution, occurring upon rocks and moss in moist places ; it has been 

 frequently collected in the mountainous districts of Vermont. 



PLATE 68. 



POLYCHIDIUM MUSCICOLUM (Sw.) Stein. 



1. Plant magnified. 



2. Section through apothecium and thallus. 



3. Paraphyses and spore-sac. 



4. Spores. 



5. Algal chain enclosed by the gelatinous covering. 



6. PSOROMA Ach. ; Michx. Fl. Bor. Am. 2: 321. 1803. 



This group differs from the preceding in its algal characters ; 

 here the algae are Cystococcus Jmmicola instead of Poly coccus punc- 

 tiformis. There is no doubt that Psoroma is phylogenetically de- 

 rived from Pannaria \ both genera are represented by the same fun- 

 gal ancestors, as is indicated by the spores, and the apothecial char- 

 acters. The reason why a given species of Pannaria should substi- 

 tute Cystococcus for Polycoccus is not known ; theoretically the con- 

 dition of affairs was perhaps as follows : the gradual changes 

 wrought in the ancestral forms of Pannaria brunnea finally adapted 

 it to enter into a more suitable symbiotic association with Cystococcus 

 .hnmicola, and as a result Psoroma hypnorum came into existence ; 



