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: 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).
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.
Sirenia (Dugongs and Manatis).
An analogous statement may be made with regard to the sea-cows, or Sirenia, which appear to be derivates from the great herbivorous order of Ungulata, and might consequently be included in that group, as indeed has been already done in Dr Max Weber's classification.
It is with the proboscidean suborder of the Ungulata to which the Sirenia are most nearly related; the nature of this relationship being described by Dr Andrews as follows: " In the first place, the occurrence of the most primitive Sirenians with which we are acquainted in the same region as the most generalized proboscidean, Moeritherium, is in favour of such a view, and this is further supported by the similarity of the brain-structure and, to some extent, of the pelvis in the earliest-known members of the two groups.
The peculiar mode of displacement of the teeth from behind forwards in some members of both groups may perhaps indicate a relationship, although in the case of the Sirenia the replacement takes place by means of a succession of similar molars, while in the Proboscidea the molars remain the same numerically, but increase greatly in size and number of transverse ridges."
These and certain other facts referred to by the same author point to the conclusion that not only are the Sirenia and the Proboscidea derived from a single ancestral stock, but that the Hyracoidea - and so Arsinoitherium - are also derivatives from the same stock, which must necessarily have been Ethiopian.
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.