There are however some lichens belonging to widely diverse genera that have retained, or reverted to, the saprophytic or parasitic habit of their fungal ancestors, though the cases that occur are generally of lichens preying on other lichens. The conditions have been described as those of “antagonistic symbiosis” when one lichen is hurtful or fatal in its action on the other, and as “parasymbiosis” when the association does little or no injury to the host. The parasitism of fungi on lichens, though falling under a different category, in many instances exhibits features akin to parasymbiosis. The parasitism of fungus on fungus is not unusual; there are instances of its occurrence in all the different classes. In the Phycomycetes there are genera wholly parasitic on other fungi such as _Woronina_ and other Chytridiaceae; _Piptocephalus_, one of the Mucorini, is another instance. _Cicinnobolus_, one of the Sphaeropsideae, preys on _Perisporiae_; a species of _Cordyceps_ is found on _Elaphomyces_, and _Orbilia coccinella_ on _Polyporus_; while among Basidiomycetes, _Nyctalis_, an agaric, grows always on _Russula_. There are few instances of lichens finding a foothold on fungi, for the simple reason that the latter are too short lived. On the perennial _Polyporeae_ a few have been recorded by Arnold[929], but these are not described as doing damage to the host. They are mostly species of _Lecidea_ or of allied genera. Kupfer[930] has also listed some 15 different lichens that he found on _Lenzites_ sp. _b._ ANTAGONISTIC SYMBIOSIS. In discussing the nutrition of lichens[931] note has been taken of the extent to which some species by means of enzymes destroy the thallus of other lichens in their vicinity and then prey on the dead tissues. A constantly cited[932] example is that of _Lecanora atriseda_ which in its early stages lives on the thallus of _Rhizocarpon geographicum_ inhabiting mountain rocks. A detailed examination of the relationship between these two plants was made by Malme and later by Bitter[933]. Both writers found that the _Lecanora_ thallus as it advanced caused a blackening of the _Rhizocarpon_ areolae, the tissues of which were killed by the burrowing slender filaments of the _Lecanora_, easily recognized by their longer cells. The invader thereafter gradually formed its own medulla, gonidial layer and cortex right over the surface of the destroyed thallus. _Lecidea insularis_ (_L. intumescens_) similarly takes possession of and destroys the thallus of Lecanora glaucoma and Malme[932] strongly suspects that _Buellia verruculosa_ and _B. aethalea_ may be living on the thallus of _Rhizocarpon distinctum_ with which they are constantly associated.