The Monodontidae family is represented by three species, the Narwhal, the Beluga and the Irrawaddy Dolphin. There are six species of Pilot and Killer Whales in the Globicephalidae family. Some taxonomists classify only two species though. The Phocoenidae family is represented by the six species of porpoise. Many people confuse dolphins and porpoises, often due to the American custom of calling dolphins porpoises.
Porpoises do belong in their own family and are quite different to a dolphin.
The final family in the suborder Odontoceti is the Platanistidae family, including five species of river dolphins. They are generally regarded as separate species though. The water-dwelling cetaceans that we know today were once land based animals. Scientists are unsure of the exact reason why the ancestors of whales, dolphins and porpoises took to the sea millions of years ago. Fossils show that their bodies adapted gradually to an aquatic way of life.