FAMILIES AND GENERA 321 



grapha) resembles Opegrapha in this respect as does also Platygrapha, a 

 genus of Lecanactidaceae, while in Graphis the paraphyses are vertical, 

 unbranched and free; Melaspilea paraphyses are somewhat similar to those 

 of Graphis. 



XVIII. ARTHONIACEAE 



The thallus of Arthoniaceae is corticolous with few exceptions and is 

 very inconspicuous, being largely embedded in the substratum. The 

 apothecia (ardellae) are round, irregular or stellate, without any margin, 

 the hymenium being protected by the dense branching of the paraphyses 

 at the tips. 



Arthonia is abundant everywhere. The species of the other genera belong 

 mostly to tropical or subtropical countries. Arthoniopsis is similar to 

 Arthonia in the character of the fruits, but the gonidium is a Phycopeltis , 

 and it is only found on leaves. Synarthonia with peculiar stromatoid fruc- 

 tification is monotypic; it occurs in Costa Rica. 



Thallus with Trentepohlia gonidia. 

 Apothecia scattered. 



Spores elongate I- or pluri-septate i. Arthonia Ach. 



Spores muri form 2. Arthothelium Massal. 



Apothecia stromatoid. 



Spores elongate, multi-septate 3. *Synarthonia Miill.-Arg. 



Thallus with Palmella gonidia. 



Spores i- or more-septate 4. Allarthonia Nyl. 



Spores muriform 5. *Allarthothelium Wain. 



Thallus with Phycopeltis gonidia. 

 Spores elongate I - or more-septate 6. *Arthoniopsis Miall.-Arg. 



XIX. GRAPHIDACEAE 



Thallus crustaceous, inconspicuous, partly immersed, mainly growing 

 on bark but occasionally on dead wood or stone. Algal cells chiefly 

 Trentepohlia, very rarely Palmella or Phycopeltis (epiphyllous). Apothecia 

 (lirellae) carbonaceous more or less linear, opening by a narrow slit with 

 a well-developed proper margin except in Gymnographa, a monotypic 

 Australian genus. In two genera, the fruit is of a compound nature, several 

 parallel discs occurring in one lirella : these are Ptychographa (on bark in 

 Scotland) and Diplogramma (Australia), both are monotypic. They must 

 not be confused with Graphis elegans and allied species in which the sterile 

 carbonaceous margin is furrowed. Two tropical genera associated with 

 Phycopeltis are epiphyllous. 



Graphidaceae are among the oldest recorded lichens, attention having 

 been drawn to them since early times by the resemblance of the lirellae on 

 the bark of trees to hieroglyphic writing. 



s. L. 21 



