IV. Distribution of Cetaceans.
Adopting the recognized division of the Cetaceans into two Suborders (Mystacoceti and Odontoceti) according as to whether their mouths are furnished with baleen ("whalebone") or teeth, we will first consider the True or Whalebone Whales, which consist of a single family Balænidæ, usually divided into five genera—Balæna, Neobalæna, Rhachianectes, Megaptera, and Balænoptera. Of these, Balæna, Megaptera, and Balænoptera are almost cosmopolitan—species of them, whether distinct or not is at present more or less uncertain, being met with in nearly every part of the ocean. But Rhachianectes has as yet been ascertained to occur only in the Northern Pacific, and Neobalæna in the South Polar Ocean; so that we have in these cases two well-marked local types to deal with.
The Toothed Whales (Odontoceti) are more diversified than the preceding group, and are usually held to embrace at least four existing families besides several extinct forms. The first family, containing the Physeteridæ, or Sperm Whales, consists of at least six genera—Physeter, Cogia, Hyperoodon, Ziphius, Mesoplodon, and Berardius. Physeter and Cogia are inhabitants of the whole oceanic area between the tropics, extending in certain localities some way beyond them. Hyperoodon is confined to the North Atlantic. Ziphius has an extensive range, and has been found in nearly every part of the ocean. Mesoplodon is also widely distributed, but is apparently more abundant in the Southern Hemisphere. Berardius, however, so far as we know at present, is restricted to the South Polar Ocean.
The third family of Toothed Whales contains only the Platanistidæ, or Freshwater Dolphins, which although, in some cases, at the present day entirely fluviatile, must necessarily have all descended from what were originally oceanic forms. The three known genera are Platanista of the Ganges and Indus, Inia of the river Amazons, and Pontoporia of the river La Plata; the last form making a connecting link between the two preceding genera and the marine Dolphins.
The fourth family of Toothed Whales, containing the Dolphins, Delphinidæ, is very numerous in species, and embraces at least fifteen or sixteen genera. But in spite of the efforts of Mr. True, who has recently given us an excellent summary of our present
