<Tr> <Th_colspan="2"> Genera </Th> </Tr> <Tr> <Td_colspan="2"> <P> See text </P> </Td> </Tr> <P> Porpoises are a group of fully aquatic marine mammals that are sometimes referred to as mereswine, all of which are classified under the family Phocoenidae, parvorder Odontoceti (toothed whales). There are six extant species of porpoise . They are small toothed whales that are very closely related to oceanic dolphins . The most obvious visible difference between the two groups is that porpoises have shorter beaks and flattened, spade - shaped teeth distinct from the conical teeth of dolphins . 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 . </P> <P> Porpoises range in size from the 1.4 metres (4.6 ft) and 54 kilograms (119 lb) vaquita, the smallest cetacean to be discovered, to the 2.3 metres (7.5 ft) and 220 kilograms (490 lb) Dall's porpoise . Several species exhibit sexual dimorphism in that the females are larger than males . They have streamlined bodies and two limbs that are modified into flippers . Dall's porpoise is one of the fastest cetaceans discovered, with the ability to travel at 41 knots (76 km / h). Porpoises have the ability to produce biosonar and it is their primary sensory system . Some species are well adapted for diving to great depths . They have a layer of fat, or blubber, under the skin to keep them warm in cold water . </P>

What is the difference between a dolphin and a porpus