Phallotorynus is a genus of livebearers in family Poeciliidae - the same live-bearing toothcarp family as guppies, mollies, and platies - restricted to river drainages of Brazil, Paraguay, and Argentina in the southern part of South America. Atlas species are tiny, 1–1.5 in, among the smallest livebearers in the atlas, and like other poeciliids the males fertilize females internally via a modified anal fin (the gonopodium) rather than scattering eggs, with females giving birth to free-swimming young.
This is a genus of mostly scientific and specialist-hobbyist interest rather than the mainstream livebearer trade that guppies and platies dominate; species such as P. dispilos (two-spot toothcarp) and P. jucundus (antler toothcarp) are known largely to South American livebearer specialists. The conservation picture is a real concern for a genus this size - of the six atlas species, three are assessed at Least Concern but two are Endangered and one Critically Endangered, and the atlas already tracks three species under CARES conservation priority, a strong signal that several of these small, range-restricted rivers are under active habitat pressure.