A plectenchymatous cortex marks some progress here also in _Leptodendriscum_, _Leptogidium_ and _Polychidium_, all of which are associated with _Scytonema_. These genera may well be derived from an elementary form such as _Thermutis_. They differ from each other in spore characters, etc., _Polychidium_ being the most highly developed with its cortex of two cell-rows and with two-celled spores. _Nostoc_ forms the gonidium of Collemaceae. In its free state it is extremely gelatinous and transmits that character more or less to the lichen. In the crustaceous genus _Physma_, which forms the base of the _Collema_ group or phylum, there is but little difference in form between the thalline warts of the lichen crust and the original small _Nostoc_ colonies such as are to be found on damp mosses, etc. In _Collema_ itself, the less advanced species are scarcely more than crusts, though the more developed show considerable diversity of lobes, either short and pulpy, or spreading out in a thin membrane. The _Nostoc_ chains pervade the homoiomerous thallus, but in some species they lie more towards the upper surface. There is no cortex, though once and again plectenchyma appears in the apothecial margin, both in this genus and in _Leprocollema_ which is purely crustaceous. _Leptogium_ is a higher type than _Collema_, the thallus being distinguished by its cellular cortex. The tips of the hyphae, lying close together at the surface, are cut off by one or more septa, giving a one- or several-celled cortical layer. The species though generally homoiomerous are of thinner texture and are less gelatinous than those of _Collema_. _c._ PYRENIDIACEAE. This small family of pyrenocarpous Phycolichens may be considered here though its affinity, through the form of the fruiting body, is with Archilichens. The gonidia are species of _Nostoc_, _Scytonema_ and _Stigonema_. There are only five genera; one of these, _Eolichen_, contains three species, the others are monotypic. The crustaceous genera have a non-corticate thallus, but an advance to lobate form takes place in _Placothelium_, an African genus. The two genera that show most development are both British: _Coriscium_ (_Normandina_), which is lobate, heteromerous and corticate—though always sterile—and _Pyrenidium_ which is fruticose in habit; the latter is associated with _Nostoc_ and forms a minute sward of upright fronds, corticate all round; the perithecium is provided with an entire wall and is immersed in the thallus. If the thallus alone were under consideration these lichens would rank with Pannariaceae.