Of the two marine mammals, porpoises are shorter and fatter (indeed, the name comes from the Latin porcopiscus, which means pig-fish) and have a blunt jaw, rather than the beak of a dolphin.
Dolphins also have a bulge at the top of the head, just in front of the blowhole, called the melon. This is filled with fat and acts as an acoustic chamber for echolocation.
Porpoises have an echolocation sense too but no melon, and are generally less acrobatic and playful than dolphins. They are both members of the toothed whales (odontoceti) and evolved into different families about 15 million years ago.
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