GENERA AND SPECIES. 157 



few or none. Disk brown. Spores few, typical, 

 11/x X 8.5/x. 



7. Parmelia perforata. Thallus large, lobes large, 

 rounded, margin somewhat crenulate, few or no soralia, 

 grayish-green, spermagonia usually numerous ; lower 

 surface black, with long black rhizoids. Apothecia 

 large, bottom of cup perforate. Disk brown. Spores 

 typical, 17/x X 10/a, 



8. Parmelia latissima. Thallus large, rounded lobes, 

 with crenate margin, and bearing white soralia, light- 

 green or grayish-green above ; lower surface black, 

 becoming brown toward margin. Apothecia rare or 

 wanting. Disk brown. Spores typical, 23.5/x X 9.5/x. 



This is evidently a tropical lichen. It is often con- 

 fused with the following species. 



9. Parmelia perlata. Thallus large, lobes rounded, 

 crenate, bearing white soralia, grayish-green ; lower 

 surface as in P. latissima. Apothecia usually want- 

 ing. Spores typical, 13/x X 9.0/x. 



This lichen yields a fine orchill. It was at one 

 time imported into England from the Canary Islands 

 as "Canary rock-moss." 



7. Cetraria. 



This genus is perhaps phylogenetically derived from 

 some Parmelioid type or types. The thallus varies 

 from foliose to typically fruticose, but in all cases the 

 lobes are flattened and broad. The amateur will no 

 doubt confound some species with Parmelia. In 

 general, however, the thallus is more brittle and dif- 

 ferent in color ; brown predominating. In some 



