Phellinus pachyphloeus (Pat.) Pat. and its allies represent a well marked group in the genus Phellinus and are characterized by the presence of setal skeletal hyphae in the context and in the dissepiments. To this group, besides P. pachyphloeus, belong P. lamaensis (Murr.) Heim in Pascalet, P. noxius (Corner) G. H. Cunn., P. melanodermus (Pat.) O. Fid., P. magnosporus (Lloyd) O. Fid. and P. portoricensis (Overh.) O. Fid. Corner (1932) presented a very detailed study of the first three species, pointing out the basic characteristics to distinguish the species. In spite of this excellent treatment many authors are still confusing such species. Information about P. lamaensis and P noxius especially is very confused in the literature. For this reason I made a special study of these and have found new information which I believe is valuable. Additional related species are placed in Phellinus.
In my opinion the genus Phellinus, having as its type P. rubriporus (Quel.) Quel. [=P- torulosus (Pers.) Bourd. & Galz.], should be restricted to yellowbrown species with a dimitic hyphal system and setae or setal skeletal hyphae. The dimitic structure is indicated by the presence of simple septate generative hyphae and skeletal hyphae. Clamp connections are never present. The basidiospores vary from hyahne to pale-brown, often being hyaline when young and darkening with age. Lowe (1957: 31) stated that P portoricensis "is unique in the colored-spored group in having setal hyphae." Its basidiospores in herbarium specimens have the same color as the basidiospores of P. pachyphloeus which, according to Corner (1932: 347), has a white spore-print which later darkens. How frequent such a phenomenon is in the yellow-brown species I do not know. Therefore, I believe that it is impossible at present to use hyaline basidiospores and yellow to yellowish-brown basidiospores as bases for distinguishing more than one genus in this group of species. I have interpreted the setal hyphae of the "pachyphloeus" group to be a structure homologous to the skeletal hyphae and analogous to the hymenial setae. Some of these hyphae differ little from the regular skeletal hyphae of P. gilvus (Schw.) Pat., as observed in P. lamaensis where they are just a little broader and clearly needle-like. The most differentiated are the setal hyphae of P. pachyphloeus which at times reach up to 850 µ long and 32 µ diam. The presence of setal hyphae does not necessarily indicate the presence of hymenial setae and vice-versa. The typical species of Phellinus possess hymenial setae. Therefore, P noxius and P melanodermus are not typical of Phellinus but, without doubt, represent the extremes of variation in the "pachyphloeus" group. For this reason they are included also here. Phellinus magnosporus may represent a link in the complex with such variation since hymenial setae are rare in this species, being present only at the base of the tubes. In an attempt to express this homology I term the setal hyphae present in the context and dissepiments as setal skeletal hyphae rather than only as setal hyphae. Both setal skeletal hyphae and hymenial setae may surpass the hymenial layer and project into the lumen of the tubes. The main difference between them is the fact that while the hymenial setae originate in the subhymenium, the setal skeletal hyphae may be produced at any place in the context and dissepiments and just by accident will enter into the lumen of the tubes. True cystidia are not present in this group. Sometimes the hymenial setae are cystidium-like and become less thickened. When this happens such a microstructure is usually paler and a little longer. It appears to m e that the variation in the frequency of the less thickened hymenial setae depends upon ecological conditions.