3. Lecanorine group (apothecia with a thalline margin).
The remaining families have all bright-green gonidia and nearly always apothecia with a thalline margin. The group includes several distinct phyla:
| XLIV. | PERTUSARIACEAE. | Thallus crustaceous. Apothecia, one or several immersed in thalline tubercles; spores mostly very large. |
| XLV. | LECANORACEAE. | Thallus crustaceous or squamulose. Apothecia mostly superficial. |
| XLVI. | PARMELIACEAE. | Thallus foliose, rarely almost fruticose or filamentous. Apothecia scattered over the surface or marginal, sessile. |
| XLVII. | USNEACEAE. | Thallus fruticose or filamentous. Apothecia sessile or shortly stalked. |
| XLVIII. | CALOPLACACEAE. | Thallus crustaceous, squamulose or minutely fruticose. Apothecia with polarilocular colourless spores. |
| XLIX. | TELOSCHISTACEAE. | Thallus foliose or fruticose. Apothecia with polarilocular colourless spores. |
| L. | BUELLIACEAE. | Thallus crustaceous or squamulose. Apothecia (lecideine or lecanorine) with two-celled, thick-walled brown spores (polarilocular in part). |
| LI. | PHYSCIACEAE. | Thallus foliose, rarely partly fruticose. Apothecia with two-celled thick-walled brown spores (polarilocular in part). |
Subclass 2. Hymenolichens.
There are only three closely related genera of Hymenolichens, Cora, Corella and Dictyonema with Chroococcus or Scytonema algae.
There is reason to dissent from the arrangement in one or two instances which will be pointed out in the following examination of families and genera.
B. Families and Genera of Ascolichens
The necessity for a well-reasoned and well-arranged system of classification is self-evident: without a working knowledge of the plants that are the subject of study no progress can be made. The recognition of plants as isolated individuals is not sufficient, it must be possible to place them in relation to others; hence the importance of a natural system. In identifying species artificial aids, such as habitat and substratum, are also often of great value, and a good working system should take account of all characteristics.
Lichen development is the result of two organisms mutually affecting each other, but as the fungus provides the reproductive system, it is the dominant partner: the main lines of classification are necessarily determined by fruit characters. The algae occupy a subsidiary position, but they also are of importance in shaping the form and structure of the thallus. The different phyla are often determined by the presence of some particular alga; it is in the delimitation of families that the algal influence is of most effect.
Zahlbruckner’s system gives due weight to the inheritance from both fungus and alga with, however, the fungus as the chief factor in development, and as his work is certain to be generally followed by modern lichenologists, it is the one of most immediate interest. His scheme has been accepted in the following more detailed account of families and genera, and for the benefit of home workers those that have not so far been recorded from the British Isles have been marked with an asterisk.