The genus _Brefeldia_ is, like some others, difficult to dispose of in any scheme of classification where linear sequence must be followed. Rostafinski placed it in an order by itself. Its relationships are on the one hand with _Amaurochaete_ and _Reticularia_, and on the other with the _Stemonitales_, though easily distinguished from either. It is intermediate to _Amaurochaete_ and _Stemonitis_, and withal, as it appears to us, a little nearer the latter, as the limits of the individual sporangia are in _Brefeldia_ pretty well defined. 1. BREFELDIA MAXIMA (_Fr._) _Rost._ PLATE V., Figs. 7, 7 _a_, 7 _b_, and PLATES XXI., XXII. 1825. _Reticularia maxima_ Fries, _Syst. Orb. Veg._, I., p. 147. 1875. _Brefeldia maxima_ (Fr.) Rost., _Versuch._, p. 8. Aethalium large, four to twenty cm, papillate above, violet-black at first, then purple or purple-brown, developed upon a widespread, silver-shining hypothallus; sporangia in favorable cases distinct, indicated above by the papillae; columellae obscure, black; capillitium abundant, the threads uniting by multifid ends to surround as with a net the peculiar vesicles; spore-mass dark violet-black, the individual spores paler by transmitted light, distinctly papillose, 12-15 mu. A very remarkable species and one of the largest, rivalled by _Fuligo_ only. To be compared with _Reticularia_, which it resembles somewhat externally, and with some of the larger specimens of _Enteridium_. The plasmodium at first white with a bluish tinge is developed abundantly in rotten wood, preferably a large oak stump, and changes color as maturity comes on, much in the fashion of _Stemonitis splendens_, leaving a widespread hypothallic film to extend far around the perfected fruit-mass. In well-matured aethalia, "_Jove favente_," the sporangia stand out perfectly distinct, particularly above and around the margins. Closely and compactly crowded, they become prismatic by mutual pressure, and attain sometimes the height of half an inch or more. In the centre of the fructification, next the hypothallus, the sporangia are very imperfectly differentiated. Many are here horizontally placed, and perhaps supplied with an imperfectly formed peridium,--if so are to be interpreted the lowest parts of the capillitial structure, the long, branching, ribbon-like strands which lie along the hypothallus. Some of these branch repeatedly with flat anastomosing branchlets, ultimately fray out into lengthened threads, and perish after all the superstructure has been blown away. From every part of the structure so described, but more especially from the margins, are given off in profusion the strange cystiferous threads, so characteristic of this genus. These are exceeding delicate filaments, attached at one end, it may be, to a principal branch, at the other free or united to a second which again joins a third, and so looping and branching, dividing, they form a more or less extended network, a capillitium in which are