174 GUIDE TO THE STUDY OF LICHENS. 



rock, perhaps rarely upon trees. The majority of so- 

 called tree Verrucarias belong to other genera. 



1. Verrucaria rupestris. Thallus deficient, dark 

 or dirty color. Apothecia small, partially immersed ; 

 perithecium black. Spores typical, 15/x X 6.5/x. 



2. Verrucaria pyrenophora. Thallus rudimentary, 

 uniformly spreading, gray. Apothecia small, raised 

 above the thallus. Disk somewhat flattened, black. 

 Spores not found. 



3. Verrucaria nigrescens. Thallus granular to in- 

 distinctly areola te, dirty gray to nearly black. Apo- 

 thecia numerous, medium size, black. Spores typical, 

 12/A X 7.5/x. 



^. Verrucaria muralis. Tliallus indistinctly areo- 

 late, light to pale brown. Apothecia numerous. Disk 

 projecting somewhat above thallus, black. Spores 

 typical, 13/A X 8/ix. 



This lichen frequently occurs on brick, mortar, lime- 

 stone, etc. 



5. Verrucaria fuscella. Thallus comparatively 

 thick, distinctly areolate, dirty gray to nearly black. 

 Apothecia small, immersed, black. Spores typical, 

 13.5/x X 6.5/x. 



6. Verrucaria margacea. Thallus thick, areolate, 

 surface smooth ; clay color. Apothecia numerous, 

 almost entirely immersed, black. Spores large, pale 

 yellowish, otherwise typical, 30/>t X 13.5/li. 



7. Pertusaria. 



Thallus crustose, but more highly developed than 

 in Verrucaria, though it varies considerably in thick- 



