The most morphologically primitive of all frogs are those with amphicoelous vertebrae. These frogs areAscaphusfrom North America, the six known fossil and Recent species ofLeiopelmafrom New Zealand, and the Jurassic fossilsNolobatrachusandVieraellafrom South America. Previous consideration of the evolutionary importance of these frogs has been almost exclusively concerned with their primitive characteristics relative to other frogs. Yet virtually no shared derived characters have been found among the genera. Genetic and cytogenetic divergence is extremely high. Present biochemical estimates of genetic divergence are at or beyond the limits of many of the techniques used. Although the living genera are very plesiomorphic,Ascaphushas such unique characteristics as an intromittent organ, high chromosome number (2n= 46), and oocyte gene amplification via eight endomitotic germinal vesicles. Derived features of RecentLeiopelmainclude inscriptional ribs, a reduced or absent tadpole stage, and low chromosome numbers (2n= 18 or 2n= 22 male/23 female).NotobatrachusandVieraellamay be difficult to assign to either Ascaphidae or Leiopelmatidae.