GENERA AND SPECIES. 163 



The Rocellas are essentially tropical, occurring on 

 rock along the sea-coast. Less frequently they occur 

 upon trees. The American specimens are found prin- 

 cipally in Lower California. 



1. Roceila tinctoria. Thallus quite large, sparingly 

 branched, lobes long, usually flattened, but often be- 

 coming rounded, gray to purplish. Apothecia com- 

 paratively rare, lateral, medium. Disk flattened to 

 slightly convex, black. Lateral white soralia. Spores 

 colorless, oblong, four-celled, 20/x X 7/x. 



This lichen occurs plentifully along the Mediter- 

 ranean coast. It occurs, in fact, in all tropical coun- 

 tries. Its uses as a dyeing agent have already been 

 referred to. It also found a use in the treatment of 

 coughs and in diseases of the kidneys. For the latter 

 purpose it was largely collected from the island of 

 Mauritius. 



2. Roceila leucophcBa. Thallus large, branching, 

 lobes flattened, gray. Apothecia lateral, medium size. 

 Disk flattened, black. Lateral white soralia. Spores 



typical, 21/x X 8/"- 



The occurrence and use of this lichen are similar to 

 those of R. tinctoria. 



10. Evernia. 



There seems to be considerable uncertainty as to 

 the limitation of this genus. As represented by the 

 two types given below, there is certainly a wide differ- 

 ence in anatomical characters. 



The thallus is generally f ruticose, large ; the branches 

 are irregularly angular and not distinctly flattened. 

 The surface of the thallus has the appearance of hav- 



