Article 6. Interpolated names.
6.1. Names of subgenera. The scientific name of a subgenus, when used with a binomen or trinomen, must be interpolated in parentheses between the generic name and the specific name; it is not counted as one of the words in the binomen or trinomen. It must begin with an upper-case letter.
Recommendation 6A. Undesirable interpolation of certain genus-group names in binomina or trinomina. No genus-group name other than a valid subgeneric name should be interpolated between a generic name and a specific name, even in square brackets or parentheses. An author who desires to refer to a former generic combination should do so in some explicit form such as "Branchiostoma lanceolatum [formerly in Amphioxus]".
6.2. Names of aggregates of species or subspecies. A specific name may be added in parentheses after the genus-group name, or be interpolated in parentheses between the genus-group name and the specific name, to denote an aggregate of species within a genus-group taxon; and a subspecific name may be interpolated in parentheses between the specific and subspecific names to denote an aggregate of subspecies within a species; such names, which must always begin with a lower-case letter and be written in full, are not counted in the number of words in a binomen or trinomen. The Principle of Priority applies to such names [Art. 23.3.3]; for their availability see Article 22.214.171.124.
Recommendation 6B. Taxonomic meaning of interpolated names. An author who wishes to denote an aggregate at either of the additional taxonomic levels mentioned in Article 6.2 should place a term to indicate the taxonomic meaning of the aggregate in the same parentheses as its interpolated species-group name on the first occasion that the notation is used in any work.
Example. In the butterfly genus Ornithoptera Boisduval, 1832 the species O. priamus (Linnaeus, 1758) is the earliest-named member of an aggregate of vicarious species that includes also O. lydius Felder, 1865 and O. croesus Wallace, 1865. The taxonomic meaning accorded to the O. priamus aggregate may be expressed in the notation "Ornithoptera (superspecies priamus)", and the members of the aggregate by the notations "O. (priamus) priamus (Linnaeus, 1758)", "O. (priamus) lydius Felder, 1865", and "O. (priamus) croesus Wallace, 1865".