136 GUIDE TO THE STUDY OF LICHENS. 



and old buildings. Some of the species are confused 

 with Theloschistes. 



1. Placodium ferrugineum. Thallus granular and 

 indistinctly areolate, variable shades of gray. Apothe- 

 cia medium, margin scarcely raised. Disk flattened, 

 rusty-brown to black. Spores typical, 17 fx X 8.5/x. 



2. Placodium cerinum. Thallus evenly spreading, 

 granular, sometimes indistinctly areolate, dark-brown. 

 Apothecia small. Disk convex, reddish-brown, margin 

 not raised. Spores typical, 12/x X 6.5/x. 



3. Placodium aurantiacum. Thallus varies from 

 evenly spreading to coarsely granular and areolate, 

 greenish-yellow or lemon color. Apothecia variable 

 in size and number. Disk flattened or slightly con- 

 vex, margin somewhat raised, reddish-brown. Spores 

 typical, 17/u, X 8/m. 



Jf. Placodium citrinum. Thallus of powdery (so- 

 redioid) granules, areolate, lemon color. Sterile. No 

 apothecia-bearing specimens have come to my notice. 

 The plant is usually mutualistically or antagonistically 

 associated with species of Lecanora^ and it may be that 

 the spores of the latter have been described as the 

 spores of P. citrinum. 



5. Placodium cinnabarinum. Thallus areolate, are- 

 oles sometimes scattered, bright-orange. Apothecia 

 small, almost entirely immersed in the areoles. Disk 

 concave, margin slightly paler than thallus. Spores 

 typical, 9/x, X 4/a. 



6. Placodium murorum. Body of thallus of convex 

 more or less irregular squamules, margin of elongated 

 bifurcate lobules, closely adnate, orange color. Apo- 



