There exists however a very large number of fully recognized parasitic microfungi belonging to various genera. Lindsay discovered many of them. Zopf[957] has given exact descriptions of a series of forms, with special reference to their effect on the host thallus. In an early paper he described a species, _Pleospora collematum_, that he found on _Physma compactum_ and other Collemaceae. The hyphae of the parasite differed from those of the host in being of a yellow colour; they did not penetrate or spread far, being restricted to rhizoid-like filaments at the base of their fruiting bodies (perithecia and pycnidia). Their presence caused a slight protuberance but otherwise did no harm to the host; the _Nostoc_ cells in their immediate vicinity were even more brightly coloured than in other parts of the thallus. In another paper[958] he gives an instance of gall-formation in _Collema pulposum_ induced by the presence of the fungus _Didymosphaeria pulposi_. Small protuberances were formed on the margins of the apothecia, more rarely on the lobes of the thallus, each one the seat of a perithecium of the fungus. No damage was done to either constituent of the thallus. _Agyrium flavescens_ grows parasitically on the under surface of _Peltigera polydactyla_. M. and Mme Moreau[959] found that the hyphae of the fungus spread between the medullary filaments of the lichen; no haustoria were observed. The mature fruiting body had no distinct excipulum, but was surrounded by a layer of dead lichen cells. It is not easy to determine the difference between parasites that are of fungal nature and those that are lichenoid; but as a general rule the fungi may be recognized by their more transient character, very frequently by their effect on the host thallus, which is more harmful than that produced by lichens, and generally by their affinity to fungi rather than to lichens. Opinions differ and will continue to differ on this very difficult question. The number of such fungi determined and classified has gradually increased, and now extends to a very long list. Even as far back as 1896 Zopf reckoned up 800 instances of parasitism of 400 species of fungi on about 350 different lichens and many more have been added. Abbé Vouaux[960] is the latest writer on the subject, but his work is mostly a compilation of species already known. He finds representatives of these parasites in nine families of Pyrenomycetes and six of Discomycetes. He leaves out of account the much debated Coniocarps, but he includes with fungi all those that have been proved to be parasymbiotic, such as _Abrothallus_.