GENERA AND SPECIES. 133 



for species of Parmelia^ otherwise there is little dif- 

 ficulty in recognizing the Physciacece. 



The apothecia belong to the thalline type, that is 

 the algaB of the thallus enter into its structure ; they 

 are discoid, rarely immersed, the disk is flattened or 

 concave, sometimes convex. 



The Physciacece seem to be better adapted to re- 

 sist extremely dry periods than most lichens. They 

 are rich in deposits of lichenic acids, which cause 

 some of the characteristic colorations in thallus and 

 apothecia. 



KEY TO THE GENERA. 



Thallus crustose, areolate, margin often lobed. 



Spores brown, thallus not bright-yellow . 1. Rinodince. 



Spores colorless, thallus yellow to orange, 2. Placodium. 

 Thallus foliose, gray, not yellow. 



Hypothecium dark 3. Pyxlne. 



Hypothecium colorless 4. Physcia. 



Thallus foliose to fruticose, often yellowish, 5. Theloschistes. 



1. Rinodina. 



This genus represents the lowest group of the 

 Physciacece, The thallus is typically crustose, gray 

 or greenish in most species ; in R. oreina the thallus 

 is of pale dirty lemon or sulphur color ; in R. chryso- 

 melcena there is also a marked yellowish tinge. In 

 some species a tendency toward the foliose type 

 is noticeable. Usually the thallus is distinctly 

 areolate and closely aduate to the substratum. The 

 areoles are bounded by a thin black zone which some 

 authors designate as the " hypothallus." 



The apothecia are immersed or sessile. In the ses- 

 sile forms the thalloid margin extends somewhiU above 



