Aquatic mammals within the infraorder Cetacea are commonly named Dolphin. Dolphin as a term, usually refers to the extant families Delphinidae also known as oceanic dolphins, Platanistidae also known as the Indian river dolphins, Iniidae also known as the New World river dolphins, Pontoporiidae also known as the brackish dolphins, and finally the extinct Lipotidae also known Chinese river dolphin. About 40 extant species are referred to as dolphins. They have variety of sizes, ranging from comparatively small 1.7-metre-long (5 ft 7 in) long and 50-kilogram (110-pound) bodied Maui’s dolphin to about 9.5 m (31 ft 2 in) and 10-tonne killer whale. They can sometimes leap about 30 feet (9.1 m). A good number of the dolphins’ species exhibit sexual dimorphism, due to the fact that the males are larger than females. Physically, their appearance is that of a streamlined body. In addition, they have two limbs that are enhanced into flippers. Some dolphins can travel at a speeds of 29 kilometres (18 miles) per hour for short distances, although, not as flexible as seals.