Although many desmostylian genera, including Behemotops katsuiei, Desmostylus hesperus, Paleoparadoxia tabatai, P. weltoni, P. repennngi, and Ashoroa laticosta are known from skulls and postcrania, Cornwallius sookensis has previously been known only from isolated teeth. New material from the Emlong Collection of the Smithsonian Institution is described here. ThNew MaterCornwallius Sookensis skulls, four partial mandibles, four isolated teeth, an innominate and a tibia from the Yaquina Formation of coastal Oregon (Zemorrian, Oligocene). The diagnosis of this taxon is emended to include the development of a postorbital process of the jugal, a dorsal midsagittal keel of the rostrum, paraglossal crests of the maxillary diastema, and a sagittal crest. Dental characteristics, including identification of canines and cusp homologies, are defined. Though the skulls and mandibles described are nearly complete, some premolars are missing and alveoli are damaged, making interpretation difficult. The adult dental formula is tentatively interpreted as 2.1.2?.3/ 1.1.1?.3, although the deciduous dentition includes two pairs of lower incisors. Mandibular characteristics include a posteriorly sloped anterior edge of the coronoid process. Phylogenetic analysis incorporating this new information resulted in one most parsimonious tree for the Order Desmostylia. The families Paleoparadoxiidae (comprising three Paleoparadoxia species) and Desmostylidae (comprised of Ashoroa, Cornwallius, Desmostylus, and Vanderhoofius) are found to be distinct clades. The clade including both species of Behemotops (Behemotopsidae) is found to be the sister group to the clade comprising Paleoparadoxiidae and Desmostylidae.