The systematics of the single-entrance, plug-shaped, soft-substrate trace fossils have long been in need of revision. At least 15 ichnogenera, comprising 32 ichnospecies, have been used, but an examination of the type material that is available suggests that this is excessive. A review of ichnotaxa indicates that the valid ichnospecies can be encompassed within five ichnogenera as follows: Conostichus (five ichnospecies), Bergaueria (including Kulindrichnus, three), Conichnus (including Amphorichnus, two), Dolopichnus (one), and Astropolichnus (one). In addition, several minor ichnogenera for related structures are considered. These are Calycraterion, Margaritichnus, and Mammillichnis.The majority of plug-shaped structures are interpreted as cubichnia or domichnia, dominantly of actinarian sea anemones. The occurrence of these trace fossils is predominantly in Paleozoic rocks.