i55 



phanic acid upon the hyphal cells, especially in the upper portion of 

 the thallus as well as in the thecium ; on account of this bitter acid, 

 various lichens of this family were used as a substitute for quinine ; 

 it is present in nearly all lichens in larger or smaller quantities. 



The apothecia are of medium size, typically discoid, and sessile 

 upon the upper surface of the thallus ; the thalloid exciple is well 

 developed. 



The distribution of the Placodiums is much as in Rinodina. They 

 seem to be somewhat more specially adapted to substrata of rock 

 but also occur upon bark, dead trees, and old boards. Like the Rino- 

 dinas they seem to be able to resist a high degree of dryness. The 

 species seem to be fairly well defined though they are sometimes 

 confused with those of Thcloschistes. 



PLATE 35. 

 Placodium elegans (Link) D.C. 



1. Portion of thallus natural size. 



2. Portion of a thallus-lobe magnified. 



3. Section of apothecium. 



4. Section of thallus. 



5. Different forms of paraphyses, and spore-sac. 



6. Spores. 



3. Pyxine Fr. PI. Homon. 267. 1825. 



The spore characters undoubtedly indicate that this genus belongs 

 to the Physciaceae. A contrasting and negative character is found 

 in the black hypothecium ; this is so marked that there is a strong 

 tendency to exclude the genus from the family. All the structural 

 characters of the thallus, as well as the apothecia and spores, point 

 to a close relationship to the Rinodinas. Pyxine is evidently also 

 closely related to Physcia; and it seems probable that it and Physcia 

 represent two different branches which proceeded from the Rino- 

 dinas. Placodium may likewise have been derived from one of the 

 Rinodinas . 



The thallus of Pyxine as represented by the only two species, 

 P. picta and P. sorediata, is large, consisting of frequently branch- 

 ing lobes. In P. picta the lobes are agglutinate and thin, and they lie 

 flat upon the substratum ; the upper surface is light grey to greenish ; 

 the lower surface is dark or black, especially in P. -picta; black rhi- 



