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.
Orders: (a) Monodelphia: Bimana, Quadrumana, Cheiroptera, Insectivora, Rodentia, Edentata, Carnivora, Amphibia, Pachydermata, Ruminantia, Cetacea; (b) Didelphia: Marsupialia, Monotremata.
Orders: Insectivora, Chiroptera, Dermoptera, Edentata (Sub-orders: Xenarthra, Pholidota, Tubulidentata), Rodentia (Sub-orders: Duplicidentata, Simplicidentata), Tillodontia, Carnivora (Sub-orders: Fissipedia, Pinnipedia, Creodonta), Cetacea (Sub orders: Archaeoceti, Odontoceti, Mystacoceti), Sirenia, Ungulata (Sub-orders: Proboscidea, Hyracoidea, Barypoda, Toxodontia, Amblypoda, Litopterna, Ancylopoda, Condylarthra, Perissodactyla, Artiodactyla), Primates (Sub-orders: Prosimiae, Anthropoidea).
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.
17 a but in none is it reduced to so great an extent as in the Cetacea, in which it is limited to a few small bristles confined to the neighbourhood of the lips and nostrils, and often present only in the young, or even the foetal condition.
Its function in the hairless Cetacea is discharged by the specially modified and thickened layer of fatty tissue beneath the skin known as " blubber."
These are absent in the Cetacea alone.
With the exception of the Cetacea, most of the Edentata, and the Sirenia, in which the teeth, when present, have been specialized in a retrograde or aberrant manner, the placental mammals as a whole have a dentition conforming more or less closely to the foregoing type.
The mouth of the great majority of mammals is peculiar for being guarded by thick fleshy lips, which are, however, absent in the Cetacea; their principal function being to seize the food, for which purpose they are endowed, as a rule, with more or less strongly marked prehensile power.
Cetacea (Whales and Dolphins) :- a.
As regards aquatic mammals, the greater number of the Cetacea, or whales and dolphins, have, as might be expected, a very wide distribution in the ocean.
The Cetacea, however, are by no means limited to the ocean, or even to salt water, some entering large rivers for considerable distances, and others being exclusively fluviatile.
The great difference in the manner of life of the sea-cows, or Sirenia, as compared with that of the Cetacea, causes a corresponding difference in their geographical distribution.