The turtle genera Chelydra (Recent, North America), Macroclemys (Recent, North America), Platysternon (Recent, Asia), Protochelydra (Paleocene, North America), and Macrocephalochelys (Pliocene, Asia) are hypothesized as a strictly monophyletic group on the basis of a shared derived character study of the skull. The five genera are therefore placed in the Family Chelydridae. The primary characters used involved the degree of temporal and cheek emargination along with changes in bone size and shape in the skull roof. Protochelydra, with the most extensive emarginations is proposed as the primitive end of a morphocline series culminating in Platysternon, which has the least emarginate condition. Cranial diagnoses for the testudinoid families Chelydridae, Emydidae, and Testudinidae are included.