Xenomystus is a genus of African knifefish in the family Notopteridae, the same family that includes the larger Papyrocranus featherbacks. The atlas's two species, X. lucani and X. nigri - both known in the atlas as the African knifefish - are found across West and Central Africa, with records from Benin, Cameroon, Liberia, Sierra Leone, Togo, Chad, the Democratic Republic of the Congo, and Gabon. Like other notopterids, Xenomystus has a laterally compressed, blade-shaped body with the anal fin fused into a single fin ribbon running back to the tail, but it is set apart from its African and Asian relatives by lacking a dorsal fin entirely.
Xenomystus nigri is a genuinely popular aquarium fish - a hardy, nocturnal knifefish that swims by undulating that long ventral fin ribbon and, like its notopterid relatives, can gulp atmospheric air to supplement gill respiration in oxygen-poor swamp and floodplain water. Atlas sizes reach about 12 in, so adults need a decently sized, well-covered tank and are best kept with robust tankmates given a semi-aggressive, territorial streak toward their own kind. X. lucani is far less documented in aquarium literature. Both atlas species are currently assessed as Least Concern.