The cetaceans or whales are widely different from all other mammals, but the tertiary Zeuglodon and Squalodon, which have been placed by some naturalists in an order by themselves, are considered by Professor Huxley to be undoubtedly cetaceans, "and to constitute connecting links with the aquatic carnivora."
Zetazeo edo baleak beste ugaztun guztiengandik oso ezberdinak dira; baina tertziarioko Zeuglodon eta Squalodon-a, naturalista batzuek biok bakarrik ordena batean jarri dituztenak, dudarik gabeko zetazeotzat jotzen ditu Huxley irakasleak, eta "uretako haragijaleekiko lokarriak direla" dio.