Amongst the six families into which fungi are divided, is one in which the spores are the principal feature, as is the aurantiaceous dust in the parasite of the goatsbeard. This family is named _Coniomycetes_, from two Greek words, meaning “dust-fungi.” This group or family includes several smaller groups, termed orders, which are analogous to the natural orders of flowering plants. Without staying to enumerate the characteristics of these orders, we select one in which the spores are enclosed in a distinct peridium, as in our typical plant they are contained within the cups. This order is the _Æcidiacei_, so called after _Æcidium_, the largest and most important of the genera included within this order. The _Æcidiacei_ are always developed on living plants, sometimes on the flowers, fruit, petioles, or stems, but most commonly on the leaves: occasionally on the upper surface, but generally on the inferior. The different species are distributed over a wide area; many are found in Europe and North America, some occur in Asia, Africa, and Australia. When the cryptogamic plants of the world shall have been as widely examined and as well understood as the phanerogamic plants have been, we shall be in a better position to determine the geographical distribution of the different orders of fungi. In the present incomplete state of our knowledge, all such efforts will be unsatisfactory. But to return to the goatsbeard, and its cluster-cups. The little fungus is called _Æcidium tragopogonis_, the first being the name of the genus, and the last that of the species. Let us warn the young student against falling into the error of supposing because in this, and many other instances, the specific name of the fungus is derived from the plant, or one of the plants, upon which it is found, that therefore the species differs with that of the plant, and that, as a rule, he may anticipate meeting with a distinct species of fungus on every distinct species of plant, or that the parasite which he encounters on the living leaves of any one plant is _necessarily_ specifically distinct from those found on all other plants. One species of _Æcidium_, for instance, may hitherto have been found only on one species of plant, whereas another _Æcidium_ may have been found on five or six different species of plants. The mycologist will look to the specific differences in the parasite without regard to the identity or distinctness of the plant upon which it is parasitic.