Of course, where later investigations have served to obliterate the once-thought patent distinctions between supposed genera or species, it is proper to unite such forms under the older determinable titles and this we have attempted. But wherever in the present work a name has been changed, the name of the earlier author will be found in parenthesis, followed immediately by that of him who made the change, and in general, recent practice, especially as expressed in the rules of the various codes, has determined the puzzling questions of nomenclature. In justification of the use of _Myxomycetes_ as a general title it may be said that in this case prevalent usage is not inconsistent with a rational application of the rules of priority. The Friesian designation _Myxogastres_ was applied by its author in 1829 to the endosporous slime-moulds as a section of gasteromycetous fungi. Four years later Link, perceiving more clearly the absolute distinctness of the group, substituted the name _Myxomycetes_. In the same year Wallroth adopted the same designation, but strangely confused the limitations of the group he named. Wallroth seems to have thought _Myxomycetes_ a synonym for _Gasteromycetes_ Fries. In 1858 DeBary applied the title _Mycetozoa_ to a group which included the then lately discovered _Acrasieae_ with the true slime-moulds, both endosporous and exosporous. For all except the _Acrasieae_ DeBary retained the old appellation, Myxomycetes. Rostafinski adopted DeBary's general name, but changed its application. As it has been shown, since DeBary's time, that the _Acrasieae_[3] have no true plasmodium, and are therefore not properly, or at least not necessarily, associated with the slime-moulds, there appears no necessity for the term _Mycetozoa_, and the question lies between _Myxogastres_ and _Myxomycetes_. Of these two names the former, as we have seen, has undoubted priority, but only as applied to the endosporous species. The same thing was true of Link's designation until DeBary redefined it, but having been taken up by DeBary, redefined and correctly applied, Myxomycetes (Link) DeBary must remain the undisputed title for all true slime-moulds, endosporous and exosporous alike. In arranging the larger divisions of the group the scheme of Rostafinski has been somewhat modified in order to give expression to what the present author deems a more natural sequence of species. The highest expression of myxomycetan fructification is doubtless the isolated sporangium with its capillitium. This is reached by successive differentiations from the simple plasmodium. The aethalium may be esteemed in some instances a case of degeneration, in others of arrested development. In any event in the present arrangement, aethalioid forms are first disposed of, leaving the sporangiate species to follow from plasmodiocarpous as directly as may be.