GENERA AND SPECIE^. Ill 



dominance of dark coloration in the Lecideas, espe- 

 cially in the disk and hypothecium. The Lecideas are 

 also more brittle. As in Biatora, the spores are sim- 

 ple, elliptical and colorless. 



The majority of the species occur upon rock, some 

 upon trees and fences. They play a very important 

 part in the disintegration of rock. 



The Lecideas have the widest range of distribution: 

 they occur at all latitudes and altitudes. Lecidca 

 geographica occurs far above the line of perpetual 

 snow ; it forms the last vegetation of the Andes, the 

 Himalayas and the arid peaks of Nova Zembla. 



1. Lecidea panceola. Thallus consists of rather 

 thick, closely crowded squamules, which are convex 

 above and of irregular outline ; ash-gray to dirty 

 brown. Apothecia comparatively large ; sometimes 

 two or three are united. Disk convex, dark-brown to 

 black. Spores simple, colorless, elliptical, 14/a X 4.5/x. 



2. Lecidea enteroleuca. Thallus granulose to in- 

 distinctly squamose, ash-gray to greenish. Apothecia 

 small to medium. Disk flattened, margin somewhat 

 raised, black. Spores simple, colorless, elliptical, 

 Ufx X 6/.. 



3. Lecidea melancheima. Thallus coarsely granu- 

 lar, rugose or warty, gray or greenish-white. Apothe- 

 cia of medium size. Disk convex, black. Hypothe- 

 cium yellowish or faintly brown. Spores colorless, 

 simple, elliptical, 9/a X 3.5yu,. 



Jf. Lecidea geographica. Thallus areolate, alter- 

 nately dark and yellow, producing a characteris'ic 

 effect faintly resembling a colored map. Apothecia 



