Zahlbruckner has included _Hydrothyria_ in this family. It is a monotypic aquatic genus found in North America and very closely allied to _Peltigera_. The British species of the genus, familiarly known as _Coccocarpia_, have been placed under _Parmeliella_, the former name being restricted to the tropical or subtropical species first assigned to _Coccocarpia_ and distinguished by the cortex, the hyphae forming it lying parallel with the surface though forming a regular plectenchyma. An Antarctic lichen _Thelidea corrugata_ with _Palmella_ gonidia is doubtfully included: the thallus is foliose, the apothecia biatorine with colourless 1-septate spores. Thallus with bright-green gonidia. With _Palmella_ 1. *Thelidea Hue. With Protococcaceae. Apothecia non-marginate (biatorine) 2. *Psoromaria Nyl. Apothecia marginate 3. Psoroma Nyl. Thallus with _Scytonema_ gonidia. Apothecia marginate, spores 1-septate 4. Massalongia Koerb. Apothecia non-marginate; spores simple. Upper surface smooth 5. *Coccocarpia Pers. Upper surface felted 6. *Erioderma Fée. Thallus with _Nostoc_ gonidia. Apothecia marginate; spores simple 7. Pannaria Del. Apothecia non-marginate; spores various. Thallus crustaceous or minutely squamulose 8. Placynthium Ach. Thallus squamulose, cortex indistinct 9. *Lepidocollema Wain. Thallus squamulose or foliose, cortex cellular 10. Parmeliella Müll.-Arg. Thallus foliose, thin veined below 11. *Hydrothyria Russ. XLII. _STICTACEAE_ Thallus foliose, mostly horizontal, with a plectenchymatous cortex on both surfaces, a tomentum of hair-like hyphae taking the place of rhizinae on the lower surface. Algal cells Protococcaceae or _Nostoc_. Cephalodia and cyphellae or pseudocyphellae often present. Apothecia superficial or lateral; spores colourless or brown, variously septate. The highly organized cortex and the presence of aeration organs—cyphellae or pseudocyphellae—which are almost solely confined to the genus _Sticta_ give this family a high position as regards vegetative development. The two genera are of wide distribution, but _Sticta_ is more abundant in the Southern Hemisphere. _Lobaria pulmonaria_ is one of our largest lichens. Under surface dotted with cyphellae or pseudocyphellae 1. Sticta Schreb. Under surface without these organs 2. Lobaria Schreb. XLIII. _PELTIGERACEAE_ A family of heteromerous foliose lichens containing in some instances blue-green (_Nostoc_), in others bright-green (Protococcaceae) gonidia, and thus representing a transition between these two series. They have large or small lobes and grow on the ground or on trees.