Thallus crustaceous squamulose. Apothecia immersed in thalline warts 1. *Calothricopsis Wain. Apothecia superficial, with thalline margin 2. *Steinera A. Zahlbr. Apothecia superficial, without a thalline margin 3. Pterygium Nyl. Thallus of small fruticose fronds. Gonidia occupying the central strand 4. *Lichinodium Nyl. Gonidia not in the centre. Apothecia immersed 5. Lichina Ag. Apothecia superficial. Paraphyses present 6. *Lichinella Nyl. Paraphyses absent 7. *Homopsella Nyl. XXXIX. _COLLEMACEAE_ The most important family of the gelatinous lichens and the most numerous. _Collema_ is historically interesting as having first suggested the composite thallus. Algal cells, _Nostoc_, which retain the chain-like form except in _Leprocollema_, a doubtful member of the family. The thallus varies from indeterminate crusts to lobes of considerable size; occasionally the lobes are narrow and erect, forming minute fruticose structures. In the more primitive genera the thallus is non-corticate, but in the more evolved, the apical cells of the hyphae coalesce to form a continuous cellular cortex, one or more cells thick, well marked in some species, in others rudimentary; the formation of plectenchyma also occurs occasionally in the apothecial tissues of some non-corticate species. The apothecia are superficial except in _Pyrenocollema_, a monotypic genus of unknown locality. They are generally lecanorine, with gonidia entering into the formation of the apothecium: in some genera they are lecideine or biatorine, being formed of hyphae alone. The spores are colourless and vary in form, size and septation.