An undescribed species may be referred to with the genus name, followed by "sp". In some cases, there is more than one undescribed species in a genus. In this case, these are often referred to by a number or letter. In the shark genus Pristiophorus, for example, there are four undescribed species, informally named Pristiophorus sp. A to D. When a formal description of one of these species is published, its temporary name will be replaced with a proper binomial name.
In bacteriology, a valid publication of a name requires the deposition of the bacteria in a Bacteriology Culture Collection. Species for which this is impossible cannot receive a valid binomial name; these species are classified as Candidatus.