Study of skulls from 145 Ribbon seals, mainly from the Bering Sea, disclosed much greater variation in form than was indicated by earlier descriptions. The characters of greatest diagnostic value in the Ribbon seal are the short rostrum, broad cranium with reduced temporal fossae. short, wide palate with small, widely spaced teeth, and the elongate, thickwalled bullae with massive, widely exposed petrosals. Comparison of these skulls with examples from each of the other recognized genera of Phocidae revealed several deficiencies in the referred cranial characteristics of taxa at all ranks below the family. Provisional support for J. E. King's recently proposed division of the Phocidae into two subfamilies, Phocinae and Monachinae, is reported; the distinctions formerly employed for diagnosis of a third subfamily, Cystophorinae, can be ascribed to convergent evolution. Craniologically, the subfamily Phocinae is divisible into three tribes: Erignathini, Cystophorini, and Phocini. The first two are monotypic; the third is polytypic. The subtribes Phocina and Histriophocina are untenable; while Histriophoca's nearest structural relative is Pagophilus, the latter shows equal or closer relationship with Phocu. Within the tribe Phocini, only the Grey seal, Hlichoerus grypus, shows sufficient cranial differentiation for recognition as a monotypic genus. The other four major taxa within that tribe show a complex system of intergrading characters that disallows generic recognition, other than as a polytypic group. Therefore, the Ribbon, Harp, Harbor, and Ringed seals are re-assigned to subgeneric rank, under the inclusive genus Phoca (sensu luto).