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 fructification is monotypic; it occurs in Costa Rica.
| Thallus with Trentepohlia gonidia. | ||
| Apothecia scattered. | ||
| Spores elongate 1- or pluri-septate | 1. | Arthonia Ach. |
| Spores muriform | 2. | Arthothelium Massal. |
| Apothecia stromatoid. | ||
| Spores elongate, multi-septate | 3. | *Synarthonia Müll.-Arg. |
| Thallus with Palmella gonidia. | ||
| Spores 1- or more-septate | 4. | Allarthonia Nyl. |
| Spores muriform | 5. | *Allarthothelium Wain. |
| Thallus with Phycopeltis gonidia. | ||
| Spores elongate 1- or more-septate | 6. | *Arthoniopsis Müll.-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.
| Thallus with Palmella gonidia. | ||
| Apothecia single. | ||
| Hypothecium dark-brown. | ||
| Spores simple | 1. | Lithographa Nyl. |
| Hypothecium colourless or brownish. | ||
| Spores colourless. | ||
| Spores simple | 2. | Xylographa Fries. |
| Spores elongate 3-8-septate | 3. | *Aulaxina Fée. |
| Spores brown. | ||
| Spores 1-septate | 4. | Encephalographa Massal. |
| Spores pluri-septate, then muriform | 5. | *Xyloschistes Wain. |
| Apothecia compound. | ||
| Spores simple, colourless | 6. | Ptychographa Nyl. |
| Spores pluri-septate, colourless | 7. | *Diplogramma Müll.-Arg. |
| Thallus with Trentepohlia gonidia. | ||
| Spores elongate 1-multi-septate, the cells longer than wide. | ||
| Spores brown. | ||
| Spores 1-(rarely more)-septate | 8. | Melaspilea Nyl. |
| Spores 3-septate (apothecia rudimentary) | 9. | *Gymnographa Müll.-Arg. |
| Spores colourless. | ||
| Spores acicular, coiled (many in the ascus) | 10. | *Spirographa A. Zahlbr. |
| Spores fusiform, straight | 11. | Opegrapha Humb. |
| Spores muriform. | ||
| Spores elongate, central cells finally muriform | 12. | *Dictyographa Müll.-Arg. |
| Spores elongate, septate, cells wider than long. | ||
| Paraphyses unbranched, filiform. | ||
| Spores multi-septate, colourless | 13. | Graphis Adans. |
| Spores multi-septate, brown | 14. | Phaeographis Müll.-Arg. |
| Spores muriform, colourless | 15. | Graphina Müll.-Arg. |
| Spores muriform, brown | 16. | Phaeographina Müll.-Arg. |
| Paraphyses clavate, warted at tips | 17. | *Acanthothecium Wain. |
| Paraphyses branched, interwoven above | 18. | *Helminthocarpon Fée. |
| Thallus with Phycopeltis gonidia (epiphyllous). | ||
| Spores elongate, 3-9-septate, colourless | 19. | *Opegraphella Müll.-Arg. |
| Spores elongate, 1-septate, brown | 20. | *Micrographa Müll.-Arg. |
XX. Chiodectonaceae
Specially distinguished in this subseries by the grouping of the somewhat rudimentary apothecia in pseudostromata in which they are almost wholly immersed. In form they are roundish or linear; the spores are septate or muriform. The thallus is thinly crustaceous and continuous: in Glyphis, Sarcographa and Sarcographina there is an amorphous upper cortex, the other genera are non-corticate. Algal cells are Trentepohlia with the exception of two epiphyllous genera associated with Phycopeltis.