Porpoises are a group of fully aquatic marine mammals, similar in appearance to a dolphin, all of which are classified under the family Phocoenidae, parvorder Odontoceti. They are, however, more closely related to narwhals and belugas than to the true dolphins. There are seven extant species of porpoise, all among the smallest of the toothed whales. Porpoises are recognised from dolphins by their flattened, spade-shaped teeth distinct from the conical teeth of dolphins, and lack of a pronounced beak, although some dolphins also lack a pronounced beak. Porpoises, and other cetaceans, belong to the clade Cetartiodactyla with even-toed ungulates, and their closest living relatives are the hippopotamuses, having diverged from them about 40 million years ago.
