The lichens of this family are rock-dwellers and are mostly to be found in hilly or Alpine regions. A tropical species, _Leptogidium dendriscum_, occurs in sterile condition in south-west Ireland. There are few species in any of the genera. Algal cells _Scytonema_. Thallus minutely fruticose, non-corticate 1. Thermutis Fr. Thallus minute, of felted filaments, cortex one cell thick 2. *Leptodendriscum Wain. Thallus of elongate filaments, cortex of several cells 3. Leptogidium Nyl. Thallus foliose or fruticose, cellular throughout 4. Polychidium Ach. Thallus crustaceous, non-corticate 5. Porocyphus Koerb. Algal cells _Stigonema_. Thallus minutely fruticose, non-corticate 6. Spilonema Born. Thallus of long branching filaments. Spores septate; paraphyses wanting 7. Ephebe Fr. Spores simple; paraphyses present 8. Ephebeia Nyl. Thallus crustaceous; upper surface non-corticate, lower surface corticate 9. *Pterygiopsis Wain. XXXVII. _PYRENOPSIDACEAE_ In this family are included gelatinous lichens of which the gonidium is a blue-green alga with a thick gelatinous coat, either _Gloeocapsa_ (including _Xanthocapsa_) or _Chroococcus_. In _Gloeocapsa_ and _Chroococcus_ the gelatinous envelope is often red, in _Xanthocapsa_ it is yellow, and these colours persist more or less in the lichens, especially in the outer layers. The thallus is in many cases a formless gelatinous crust of hyphal filaments mingling with colonies of algal cells as in _Pyrenopsis_; but small fruticose tufts are characteristic of _Synalissa_, and larger foliose and fruticose thalli appear in some exotic genera. A plectenchymatous cortex is formed on the thallus of _Forssellia_, a crustaceous genus from Central Europe, with two species only; the whole thallus is built up of a kind of plectenchyma in some others, but in most of the genera there is no tissue formed. The apothecia, as in Ephebaceae, are generally half-closed.