The fern flora of eastern North America attains its maximum development in the number of species as well as in the luxuriant abundance of many of them, in the northeastern states and in the Appalachian Mountains. The number of species diminishes gradually westward, and most of them have disappeared in the plains states, although some few extend to the Rocky Mountains, where they meet many species from the west and south, and a few of the latter range eastward as far as the Mississippi Valley. The State of Missouri, situated about half-way between the northern and southern boundaries and somewhat east of the geographical center of the United States, embraces the western range limit of several species of ferns and fern allies. Both on account of its central geographical position and of the great diversity of its topography and of its rocks and soils, the fern flora of the state is somewhat richer and more diverse than that of most of the surrounding states. Three distinct topographical regions are found in the state, and several more or less well-marked subdivisions of these might be recognized on the basis of geological formations and soils. The three main divisions from