118 GUIDE TO THE STUDY OF LICHENS. 



concave, reddish-brown, margin somewhat rugose. 

 Spores multilocular, somewhat colored when mature, 



40/A X 20/x. 



IJf. Psora. 



By some authors this genus is classed with Biatora, 

 others class it with Lecanora. It is the first group 

 of the LecideacecB in which the thallus is typically 

 foliose. The thallus is simple, entire, small, and is 

 usually closely adnate to the substratum, or the entire 

 lobes may be plicate or ascending. The lobes never 

 became large, scarcely exceeding one-fourth of an 

 inch in diameter. They are, however, comparatively 

 thick, thus giving them considerable rigidity. Their 

 color is quite variable ; the upper surface varies from 

 light-gray to black ; the lower surface is usually of a 

 lighter color. 



The apothecia are small to medium, distributed 

 over the thallus, but sometimes ranged about the 

 margin. They are somewhat discoid and more or less 

 immersed in the thallus. The disk is dark-colored 

 (rusty). The spores are simple, elliptical, colorless, 

 closely resembling those of Biatora. 



The Psoras occur upon rock and soil ; they seem to 

 be quite generally distributed. 



1. Psora icteria. Thallus-lobes rather small, 

 scarcely ascending, margin somewhat crenate and 

 light colored ; upper surface greenish becoming 

 tawny with age ; light colored beneath. Apothecia 

 rather small, sessile. Disk nearly black. Thecium 

 and hypothecium yellowish. Spores simple, color- 

 less, elliptical, 16/x X 7/x. 



