The otarioid seals originated in the North Pacific region and most of their history is centered in this area. They include four families: the extant Odobenidae (walruses) and Otariidae (fur seals and sea lions) and the extinct Desmatophocidae and Enaliarctidae. Early in their development, the odobenids dispersed through the Central American Seaway to the North Atlantic, where the living walrus evolved. By late Pliocene time, the odobenids had become extinct in the North Pacific but the modern walrus spread from the Atlantic to the Pacific by way of the Arctic Ocean in late Pleistocene time. The history of the odobenids is known back to the early middle Miocene, at which time they seem to have evolved out of the ancestral otarioid family, the Enaliarctidae. The odobenids were most 1 diverse in the North Pacific during the late Miocene when six genera belonging to two subfamilies are known. A new genus and species, Aiuukus cedrosensis, from the Almejas Formation of Baja California, is a privitive odobenid of the Subfamily Odobeninae. Three Pliocene and younger genera, including the living genus, are recognized in the Atlantic, but evolution in this area was essentially unidirectional toward modern walrus. The history of the otariid seals began in the late middle or early late Miocene when this family evolved out of the last ofthe Enaliarctidae. The otariids remained a family with little variety, and, relative to the odobenids, slight evolutionary change until the late Pliocene or early Pleistocene, more or less the time of extinction of the odobenids in the Pacific. During this period of little diversification, one new otariid genus, Thallassoleon, is recognized. This genus contains two new species: T. mexicanus, from the Almejas Formation of Baja California, and T. mac• nallyae, from the Drakes Bay Formation of Point Reyes, California. The otariids are today in their period of greatest diversification-there are seven living genera. By early Pliocene time, the otariids had dispersed to the South Pacific Ocean, whence during the Pleistocene, they spread to their present circumantarctic distribution. They have never reached the North Atlantic. The extinct desmatophocids evolved out of the enaliarctid group in the early Miocene and became most diverse in the middle Miocene. They are last know.n from rocks of late Miocene age, at about the time that the odobenids began to diversify. The desmatophocids seem nev~r to have left the North Pacific, but they are known from southern California to Alaska and from Japan. The Enaliarctidae were ancestral to the other three families and are largely unstudied. They were derived from primitive ursid land carnivores, presumably during the Oligocene, and may be described as flippered marine carnivores with heterodont dentition in which the premolar, carnassial, and molar teeth are differ
