The structure of living and fossil marattiaceous ferns is compared organographically. Stelar structure exhibits a basic pattern in all genera in spite of modifications correlated with size and symmetry. Fronds exhibit gross morphological and anatomical differences but can likewise be interpreted as specializations associated with size and form, and genera can be related to one another in an evolutionary sequence. Living genera with dorsiventral stems and once pinnate or palmate fronds are considered derived from fossil forms with radially symmetrical stems and large, highly branched fronds. Primitive fronds had small pinnules which became modified into large laminar units in the living genera. Evidence from fossil and living genera is cited in support of these conclusions. The problem of sporangial aggregation is discussed in light of transformations in pinnule morphology. The linear sorus in the Pennsylvanian age Eoangiopteris is considered to have been produced on pinnatifid areas of Psaroniustype fronds. Other regions of the frond, or perhaps different fronds or plants, bore radially symmetrical synangia identified as Scolecopteris iowensis. The significance of this interpretation is the possible insight provided relative to the evolution of large laminar units and concomitant sporangial aggregation and distribution. Senftenbergia is rejected as a marattiaceous fructification. The Marattiales is a distinct group of ferns restricted to tropical areas and consequently known to most North Americans only through occasional contact in conservatories or herbaria. The order is considered primitive among living ferns and, primarily because of their eusporangiate development, usually placed close to the Ophioglossales in taxonomic schemes. Most of the detailed studies of living genera were undertaken in the early years of this century, largely under the stimulus provided by the occurrence of fossil marattiaceous plants in Carboniferous strata. In recent years our knowledge of the fossil forms has accumulated in the virtual absence of further work with the extant genera so that once again we are in a position of lacking comparative data from the living representatives. Seven extant genera have been described: Angiopteris, Marattia, Archangiopteris, Protomarattia, Macroglossum, Christensenia, and Danaea. Of these, Angiopteris, Marattia, Danaea, and Christensenia are the best known. Relatively little is known of Archangiopteris and Macroglossum, which contain only six species between them. No anatomical studies have been undertaken on Protomarattia, and it would be helpful to know how this genus compares with the other genera. (Copeland, 1947, in his Genera Filicum follows Christensen and Tardieu and places Protomarattia tonkinensis in synonymy with Archangiopteris tamdaoensis.) The group is usually characterized as consisting of large, coarse, sappy ferns, the latter feature being a function of numerous mucilage canals and tannin cells in the fundamental tissue. The vascular anatomy, at least in the larger stems, is very complex, consisting of several interconnected cycles composed of small meristeles. Stems of some genera-Angiopteris, Marattia, and Macroglossumare short, erect, tuberous and occasionally up to two feet in diameter. The remaining genera have trailing, mostly dorsiventral stems. All possess large