The porpoises are small cetaceans of the family Phocoenidae; they are related to whales and dolphins.
They are distinct from dolphins, although the word "porpoise" has been used to refer to any small dolphin, especially by sailors and fishermen.
The most obvious visible difference between the two groups is that porpoises have spatulate (flattened) teeth distinct from the conical teeth of dolphins.
In addition, porpoises are relatively r-selected compared with dolphins: that is, they rear more young more quickly than dolphins.
Porpoises, divided into six species, live in all oceans, mostly near the shore.
Probably best known is the Harbour Porpoise, which can be found across the Northern Hemisphere.
Porpoises tend to be smaller but stouter than dolphins.
They have small, rounded heads and blunt jaws instead of beaks.
Their teeth are spade-shaped, whereas dolphins have conical teeth.
In addition, a porpoise's dorsal fin is generally triangular, rather than falcate (curved) like that of many dolphins and large whales.