3.1. The principal ranks of taxa in descending sequence are: kingdom (regnum), division or phylum (divisio, phylum), class (classis), order (ordo), family (familia), genus (genus), and species (species). Thus, except for some fossil plants (see Art. 3.3), each species is assignable to a genus, each genus to a family, etc.
3.2. The principal ranks of nothotaxa (hybrid taxa) are nothogenus and notho-species. These are the same ranks as genus and species, only the terms denoting the ranks differing in order to indicate the hybrid character (see App. I).
3.3. Because of the fragmentary nature of the specimens on which the species of some fossil plants are based, the genera to which they are assigned are not assignable to a family, although they may be referable to a taxon of higher rank. Such genera are known as form-genera (forma-genera).
Ex. 1. Form-genera: Dadoxylon Endl. (Coniferopsida), Pecopteris (Brongn.) Sternb. (Pteridopsida), Stigmaria Brongn. (Lepidodendrales), Spermatites Miner (seed-bearing plants).
Ex. 2. The following are, however, not form-genera: Lepidocarpon D. H. Scott (Lepidocarpaceae), Mazocarpon M. J. Benson (Sigillariaceae), Siltaria Traverse (Fagaceae).
Note 1. Art. 59 provides for form-taxa for asexual forms (anamorphs) of certain pleomorphic fungi, at any rank.
3.4. As in the case of certain pleomorphic fungi, the provisions
of this Code do not prevent the publication and use of
names of form-genera of fossils.