The filamentous structure appears abruptly, unless we consider it as foreshadowed by _Parmelia pubescens_. The base is secured by strong sheaths of enduring character; tensile strains are provided for either by a chondroid axis, as in _Usnea_, or by cortical development, as in _Alectoria_; the former method of securing strength seems to be the most advantageous to the plant as a whole, since it leaves the outer structures more free to develop, and there is therefore in _Usnea_ a greater variety of branching and greater growth in length, which are less possible with the thickened cortex of _Alectoria_. _ee._ _PHYSCIACEAE._ There remains still an important phylum of Lecanorales well defined by the polarilocular spores[1014]. It also arises from a _Biatora_ species and forms a parallel development. Even in this phylum there are two series: one with colourless spores and mostly yellow or reddish either in thallus or apothecium, and the other with brown spores and with cinereous-grey or brown thalli. The dark spores are in many of the species typically polarilocular, though in some the median septum is not very wide and no canal is visible. Practically all of the lighter coloured forms contain parietin either in thallus or apothecia or in both; it is absent in the dark-spored series. Among the lighter coloured forms it is difficult to decide which of these two striking characteristics developed first, the acid or the peculiar spore. Probably the acid has the priority: there is one common rock lichen in this country, _Placodium rupestre_ (_Lecanora irrubata_), which gives a strong red acid reaction with potash, but in which the spores are still simple, and the fruit structure in the biatorine stage. Another species, _Pl. luteoalbum_, with a purplish reaction in the fruit only, shows septate spores but with only a rather narrow septum. The development continues through biatorine forms to lecanorine with a fully formed thalline margin. Among these latter we encounter _Pl. nivale_ which is well provided with acid but in which the spores have become long and fusiform with little trace of the polar cells or central canal. We must allow here also for reversions, and wanderings from the straight road. From crustaceous the advance is normal and simple to squamulose forms which in this phylum maintain a stiff regularity of thalline outline termed “effigurate”; the squamules, developing from the centre, extend outwards in a radiate-stellate manner. There are also foliose thalli in the genus _Xanthoria_ and fruticose in _Teloschistes_. The cortex in the former horizontal genus is of plectenchyma, and no peculiar structures have emerged. In _Teloschistes_ the cortex is of compact parallel hyphae (fibrous) which form the strengthening structure of the narrow compressed fronds (_T. flavicans_).