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An extinct genus of marine mammals, regarded by Huxley as intermediate between the true cetaceans and the carnivorous seals. The species had an elongated snout, conical incisors, and molar teeth with triangular serrated crowns, implanted in the jaws by two roots, each molar appearing to be formed of two separate teeth united at the crown (whence the generic name). They belong to the eocene and miocene, the best known species being Z. cetoides of the middle eocene of the United States, which attained a length of 70 feet. The first found remains were believed to be those of a reptile, and the name Basilosaurus was therefore given to them.