=Group 3.= Brightly coloured acids, yellow, orange or red, all derivatives of pulvinic acid (C₁₈H₁₂O₅), a laboratory compound which has not been found in nature. The group includes among others vulpinic acid (C₁₉H₁₄O₅) from the brilliant yellow _Evernia_ (_Letharia_) _vulpina_, stictaurin (C₃₆H₂₂O₉) deposited in orange-red crystals on the hyphae of _Sticta aurata_, and rhizocarpic acid (C₂₆H₂₀O₆) chiefly obtained from _Rhizocarpon geographicum_: it crystallizes out in slender citron-yellow prisms. =Group 4.= Only one acid, usninic (C₁₈H₁₆O₇), a derivative of acetylacetic acid, is placed in this group. It is of very wide-spread occurrence, having been found in at least 70 species belonging to very different genera and families of crustaceous shrubby and leafy lichens. Zopf himself isolated it from 48 species. =Group 5.= The thiophaninic acid (C₁₂H₆O₉) group representing only a small number. They are all sulphur-yellow in colour and soluble in alcohol, the solution becoming blackish-green or dirty blue on the addition of iron chloride, with one exception, that of subauriferin obtained from the yellow-coloured medulla of _Parmelia subaurifera_ which stains faintly wine-red in an iron solution. Thiophaninic acid, which gives its name to this group, occurs in _Pertusaria lutescens_ and _P. Wulfenii_, both of which are yellowish crustaceous lichens growing mostly on the trunks of trees. II. LICHEN-ACIDS OF THE BENZOLE SERIES The larger number of lichen-acids belong to this series, of which 94 at least are already known. They are divided into two subseries: I. Orcine derivatives, and II. Anthracene derivatives. SUBSERIES I. ORCINE DERIVATIVES Zopf specially insists that the grouping of this series must be regarded as only a provisional arrangement of the many lichen-acids that are included therein. All of them are split up into orcine and carbonic acid by ammonia and other alkalies. On exposure to air, the ammoniacal or alkaline solution changes gradually into orceine, the colouring principle and chief constituent of commercial orchil. Orcine is not found free in nature. The orcine subseries includes five groups: =Group 1.= The substances in this group form, with hypochlorite of lime (“CaCl”), red-coloured compounds which yield, on splitting, orsellinic acid. Zopf enumerates seven acids as belonging to this group, among which is lecanoric acid (C₁₆H₁₄O₇), found in many different lichens, _e.g._ _Roccella tinctoria_, _Lecanora_ tartarea, etc.: whenever there is a differentiated pith and cortex it occurs in the pith alone. Erythrin (C₂₀H₂₂O₁₀), a constituent of the British marine lichen _Roccella fuciformis_, also belongs to this orsellinic group.