Farjon (1989) provides a thorough taxonomic review of the genus.

Some list as few as five subspecies (excluding nominal), but a taxonomic review is needed and most recognise more:

The genus needs in a taxonomic review of species.

Mr. Spiegelman said he was vexed that The Times judged his book to be fiction and wrote a letter to the editor asking for a taxonomic review.

A taxonomic review of the Pachycephalosauridae (Dinosauria: Ornithischia).

A taxonomic review of A. eyrei is under way.

A taxonomic review of the genera Eriostemon and Philotheca (Boronieae: Rutaceae).

The species needs taxonomic review for a number of reasons.

This genus has been strongly suggested to be put under taxonomic review.

This was in a taxonomic review of the Notostraca that also supported using the genus name Triops instead of Apus.