What Is The Difference Between Porpoises And Dolphins?
There are about 40 species of marine dolphins (family Delphinidae) and porpoises (family Phocoenidae), combined. The chief differences between dolphins and porpoises are in their snouts and teeth. A true dolphin has a beak-like snout and cone-shaped teeth. A true porpoise has a rounded snout and flat or spade-shaped teeth.
Sources: Macmillan Illustrated Animal Encyclopedia, pp. 112, 114; Nowak, Ronald M. Walker's Mammals of the World, 5th ed., vol. 2, pp. 969-82.