The thallus of Dirinaceae is crustaceous, while the genera of Roccellaceae are mostly of an advanced fruticose type, though in one, _Roccellina_, there is a crustaceous thallus with an upright portion consisting of short swollen podetia-like structures with apothecia at the tips; and in another, _Roccellographa_, the fronds broaden to leafy expansions. They are nearly all rock-dwellers, often inhabiting wind-swept maritime coasts, and a strong basal sheath has been evolved to strengthen their foothold. In some genera the sheath contains gonidia; in others the tissue is wholly of hyphae—in nearly every case it is protected by a cortex. In the upright fronds the structure is radiate: generally a rather loose strand of hyphae more or less parallel with the long axis of the plant forms a central medulla. The gonidia lie outside the medulla and just within the outer cortex. The latter, in a few genera, is fibrous, the parallel hyphae being very closely compacted; but in most members of the family the fastigiate type prevails, as in the allied family Dirinaceae. _d._ THALLUS OF CYCLOCARPINEAE. This is by far the largest and most varied series of Archilichens. It is derived, as regards the fungal constituent, from the Discomycetes, but in these fungi, the vegetative or mycelial body gives no aid to the classification which depends wholly on apothecial characters. In the symbiotic condition, on the contrary, the thallus becomes of extreme importance in the determination of families, genera and species. There has been within the series a great development both of apothecial and of thalline characters in parallel lines or phyla. _AA._ _LECIDEALES._ The type of fruit nearest to fungi in form and origin occurs in the Lecideales. It is an open disc developed from the fungal symbiont alone, the alga taking no part. There are several phyla to be considered. _aa._ _COENOGONIACEAE._ There are two types of gonidial algae in this family, and both are filamentous forms, _Trentepohlia_ in _Coenogonium_ and _Cladophora_ in _Racodium_. The resulting lichens retain the slender thread-like form of the algae, their cells being thinly invested by the hyphae and both symbionts growing apically. The thalline filaments are generally very sparingly branched and grow radially side by side in a loose flat expansion attached at one side by a sheath, or the strands spread irregularly over the substratum. Plectenchyma appears in the apothecial margin in _Coenogonium_. Fruiting bodies are unknown in _Racodium_. Coenogoniaceae are a group apart and of slight development, only the one kind of thallus appearing; the form is moulded on that of the gonidium, and is, as Reinke[1005] remarks, perfectly adapted to receive the maximum of illumination and aeration. _bb._ _LECIDEACEAE AND GYROPHORACEAE._ The origin of this thalline phylum is distinct from that of the previous family, being associated with a different type of gonidium, the single-celled alga of the Protococcaceae.