One of the largest lichen families as regards both genera and species, and of world-wide distribution. The algal cells are Protococcaceae. The thallus is mostly crustaceous but it becomes squamulose in Psora, a section of Lecidea; and in Sphaerophoropsis, a Brazilian genus, there are small upright fronds or stalks with lateral apothecia. The prevailing colour of the thallus is some shade of grey, but it ranges from white or yellow to dark-brown or almost black. Cephalodia appear in some of the species.
The apothecia have a proper margin only, no gonidia taking part in the fruit-formation. They may be soft and waxy (biatorine) or hard and carbonaceous (lecideine). The genera are mainly based on spore characters which are very varied.
The arrangement of genera given below follows that of Zahlbruckner; in several instances, both as to the limitations of genera and to the nomenclature, it differs from that of British text-books, though the general principle of classification is the same.
| Thallus crustaceous non-corticate. | ||
| Spores simple. | ||
| Spores small, thin-walled. | ||
| Spores colourless | 1. | Lecidea Ach. |
| Spores brown | 2. | *Orphniospora Koerb. |
| Spores large, thick-walled | 3. | Mycoblastus Norm. |
| Spores 1-septate. | ||
| Spores small, thin-walled | 4. | Catillaria Th. Fr. |
| Spores large, thick-walled | 5. | Megalospora Mey. and Flot. |
| Spores elongate, 3-multi-septate. | ||
| Spores elongate, narrow, thin-walled | 6. | Bacidia A. Zahlbr. |
| Spores elongate, large and thick-walled | 7. | Bombyliospora De Not. |
| Spores muriform. | ||
| Spores colourless; on trees | 8. | Lopadium Koerb. |
| Spores colourless to brown; on rocks | 9. | Rhizocarpon Th. Kr. |
| Thallus warted or squamulose, corticate. | ||
| Spores elongate, 1-7-septate, thin-walled | 10. | Toninia Th. Fr. |
| Thallus of upright podetia-like small fronds. | ||
| Spores ellipsoid, becoming 1-septate | 11. | *Sphaerophoropsis Wain. |
XXXII. Phyllopsoraceae
A small family of exotic lichens with a somewhat more developed thallus than that of the Lecideaceae, being in both of the genera squamulose or almost foliose.
The apothecia are without a thalline margin; they are biatorine or lecideine; the hypothecium is formed of plectenchyma and is purple-red in one species, Phyllopsora furfuracca. The two genera differ only in spore characters. There are fifteen species, mostly corticolous, belonging to Phyllopsora; only one, from New Zealand, is recorded for Psorella.
| Spores simple | 1. | *Phyllopsora Müll.-Arg. |
| Spores elongate, septate | 2. | *Psorella Müll.-Arg. |