The distinctive characteristics of every genus and species belonging to, or related to, the family, Whaitsiidae, are described. Special attention is given to interrelationship. Ten genera and twenty species are described or discussed, including one new genus and five new species. Attempts are made to illustrate their relationship in the form of a genealogical tree and the genera are discussed in the orderPromoschorhynchus, Moschorhynchus, WhaitsiaandNotosollasia, starting with the primitive new genus and proceeding in the order of specialization. The relationship of the genusMoschorhinusis then discussed and thereafter the incompletely known generaAlopecopsis, Notaelurops, Aneugomphius, TheriognathusandHyenosaurus.The new species described arePromoschorhynchus plalyrhinus, Moschorhynchus brachyrhinusandM. gracilis, Notosollasia boonstraiandN. longiceps.