Of the marine orders of Sirenia and Cetacea the Dugong, Halicore, is exclusively found in the Indian Ocean; and a dolphin, Platanista, peculiar to the Ganges, ascends that river to a great distance from the sea.
The cetacea and a few rodents are also without it.
There are no large mammals in all this extensive region except the Cetacea and a species of the Phocidae of southern waters.
The Cetacea, which frequent these southern waters, are represented by four species - two dolphins and the sperm and right whale - and the Phocidae by six species, one of which (Phoca lupina) differs but little from the common seal.