The more elementary species are of extremely simple structure as exemplified in such species as _Lecidea_ (_Biatora_) _uliginosa_ or _Lecidea granulosa_. These lichens grow on humus-soil and the thallus consists of a spreading mycelium or hypothallus with more or less scattered thalline granules containing gonidia, but without any defined structure. The first advance takes place in the aggregation and consolidation of such thalline granules and the massing of the gonidia towards the light, thus substituting the heteromerous for the homoiomerous arrangement of the tissues. The various characters of thickness, areolation, colour, etc. of the thallus are constant and are expressed in specific diagnoses. Frequently an amorphous cortex of swollen hyphae provides a smooth upper surface and forms a protective covering for such long-lived species as _Rhizocarpon geographicum_, etc. The squamulose thallus is well represented in this phylum. The squamules vary in size and texture but are mostly rather thick and stiff. In _Lecidea ostreata_ they rise from the substratum in serried rows forming a dense sward; in _L. decipiens_, also a British species, the squamules are still larger, and more horizontal in direction; they are thick and firm and the upper cortex is a plectenchyma of cells with swollen walls. Solitary hyphae from the medulla pass downwards into the support. Changes in spore characters also arise in these different thalline series, as for instance in genera such as _Biatorina_ and _Buellia_, the one with colourless, the other with brown, two-celled spores. These variations, along with changes in the thallus, are of specific or generic importance following the significance accorded to the various characters. In one lichen of the series, the monotypic Brazilian genus _Sphaerophoropsis stereocauloides_, the thallus is described by Wainio[1006] as consisting of minute clavate stalks of interwoven thick-walled hyphae, with gelatinous algae, like _Gloeocapsa_, interspersed in groups, though with a tendency to congregate towards the outer surface. The highest development along this line of advance is to be found in the Gyrophoraceae, a family of lichens with a varied foliose character and dark lecideine apothecia. The thallus may be monophyllous and of fairly large dimensions or polyphyllous; it is mostly anchored by a central stout hold-fast and both surfaces are thickly corticate with a layer of plectenchyma; the under surface is mostly bare, but may be densely covered with rhizina-like strands of dark hyphae. They are all northern species and rock-dwellers exposed to severe extremes of illumination and temperature, but well protected by the thick cortex and the dark colouration common to them all.