Bentham's arrangement stood until 1996, when Alex George published a revision of the genus.

It was finally described in Alex George's 1981 revision of the genus.

This comprehensive revision of the genus has now been widely accepted.

By the time a full revision of the genus was published in 1963, the genus included about 50 species.

There are four described species, although a revision of the genus is being carried out which will add at least four new species.

The species most recently recognised in a revision of the genus are:

Various authorities accept some or all the systematics previously applied, but most agree a revision of the genus is needed.

This phylogenetic study has made the need for a revision of the genus even clearer.

In 1955, he published a revision of the genus.

Zona's revision of the genus concluded that they both belonged to the same species.