The tailed frogs are two species of frogs in the genus Ascaphus, the only taxon in the family Ascaphidae. The "tail" in the name is actually an extension of the male cloaca. The tail is one of two distinctive anatomical features adapting the species to life in fast-flowing streams. These are the only North American frog species that reproduce by internal fertilization. They are among the most primitive known families of frogs.