58.1. A name rejected or otherwise unavailable for use under Art. 51, 52, 53, 54, or 56, 57, 58 is replaced by the name that has priority (Art. 11) in the rank concerned. If none exists in any rank a new name must be chosen: (a) the taxon may be treated as new and another name published for it, or (b) if the illegitimate name is a later homonym, an avowed substitute (nomen novum) based on the same type as the rejected name may be published for it. If a name is available in another rank, one of the above alternatives may be chosen, or (c) a new combination, based on the name in the other rank, may be published.
58.2. Similar action is to be taken if transfer of an epithet of a legitimate name would result in a combination that cannot be validly published under Art. 21.3, 22.4, 23.4 or 27, or in a later homonym.
Ex. 1. Linum radiola L. (1753) when transferred to Radiola Hill may not be named "Radiola radiola", as was done by Karsten (1882), since that combination is invalid (see Art. 23.4 and 32.1(b)). The next oldest name, L. multiflorum Lam. (1779), is illegitimate, being a superfluous name for L. radiola. Under Radiola, the species has been given the legitimate name R. linoides Roth (1788).
58.3. When a new epithet is required, an author may adopt the epithet of a previous illegitimate name of the taxon if there is no obstacle to its employment in the new position or sense; the resultant combination is treated as the name of a new taxon or as a nomen novum, as the case may be.
Ex. 2. The name Talinum polyandrum Hook. (1855) is illegitimate, being a later homonym of T. polyandrum Ruiz & Pav. (1798). When Bentham, in 1863, transferred T. polyandrum Hook. to Calandrinia, he called it C. polyandra. This name is treated as having priority from 1863, and is cited as C. polyandra Benth., not C. polyandra (Hook.) Benth.
Ex. 3. While describing Collema tremelloides var.
cyanescens, Acharius (Syn. Meth. Lich.: 326. 1814) cited
C. tremelloides var. caesium Ach. (Lichenogr. Universalis:
656. 1810) in synonymy, thus render-ing his new name illegitimate.
The epithet cyanescens was taken up in the combination
Parmelia cyanescens Schaer. (1842), but this is a later
homonym of P. cyanescens (Pers.) Ach. (1803). In Collema,
however, the epithet cyanescens was available for use,
and the name C. cyanescens Rabenh. (1845), based on the
same type, is legitimate. The correct author citation for Leptogium
cyanescens, validated by Körber (1855) by reference to
C. cyanescens "Schaer.", is therefore (Rabenh.)
Körb., not (Ach.) Körb. nor (Schaer.) Körb.
58A.1. Authors should avoid adoption of the epithet of
an illegitimate name previously published for the same taxon.