« AnteriorContinuar »
SECTION III.—DISTRIBUTION OF SIRENIANS
Only two forms of Sirenians are at the present time existing on the earth's surface—the Manatee (Manatus)
and the Dugong (Halicore)—each representing a distinct family of the Order. The Manatee (Fig. 44, p. 202) is an inhabitant of the coasts and estuaries of both sides of the middle Atlantic Ocean-one species (Manatus senegalensis) occurring on the African shores, and another (M. americanus) on the South American coast and in the Antilles. A third species (M. inunguis), so far as we know at present, is found only in fresh water high up the Amazon.
The Dugong (Halicore) (Fig. 45, p. 203) is distributed from East Africa, along the shores of the Indian Ocean and its islands, to North Australia. Three species of this genus have been established-Halicore tabernaculi from the Red Sea, H. dugong from the Indian Ocean, and H. australis from Australia; but it is doubtful how far these forms are actually distinguishable.
Besides Manatus and Halicore, a third quite distinct form of Sirenian was formerly an inhabitant of the North Pacific. This was Steller's Sea-cow (Rhytina stelleri), by far the largest animal of the group, which was exterminated by human agency about 1768. Fortunately recent researches in Behring's Island have been successful in supplying specimens of its skeleton for our principal museums, and Steller, its discoverer, left to posterity a good account of its habits and anatomy.
SECTION IV.-DISTRIBUTION OF CETACEANS
Adopting the recognized division of the Cetaceans into two sub-orders, 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 Balanidæ, usually divided into five genera: Balæna, Neobalena, Rhachianectes, Megaptera, and Balænoptera. Of these,
Balæna (Fig. 46, p. 204), 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 extinct forms. The first family, containing the Physeteridæ or SpermWhales, consists of at least six genera, Physeter (Fig. 47, p. 205), 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 contined 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,
FIG. 48.—THE SEE-SEE.
however, so far as we know at present, is restricted to the Pacific Ocean.
The second (existing) family of Toothed Whales contains only the Platanistidæ, or Freshwater Dolphins, which although, in some cases, at the present day entirely fluviatile, must probably have descended from oceanic forms.? The three known genera are Platanista of the Ganges and Indus (Fig. 48), Inia of the river Amazon, and Pontoporia of the river La Plata; the last form making
i Sir William Flower ("Whales, Past and Present,” Proc. Roy. Inst., x., p. 360, 1883) rather favours the idea of a freshwater origin of the Cetaceans.
a connecting link between the two preceding genera and the marine Dolphins.
The third family of Toothed Whales, containing the Dolphins, Delphinidæ, is very numerous in species and embraces at least fifteen or sixteen genera, of which the Common Dolphin (Fig. 49) is a good example. But in spite of the efforts of Mr. True, who has recently given us an excellent summary of our present knowledge of them both the genera and species of Delphinidæ are still so imperfectly understood that not much can be said about their geographical distribution. Most of the forms
FIG. 49.—THE COMMON DOLPHIN.
appear to be very widely distributed, but it may be said generally that Dolphins are most abundant in the intertropical seas and less plentiful both to the north and south of them.
There are, however, two forms that are exclusively inhabitants of the Northern Oceans. These are the very remarkable Narwhal (Monodon), in which the male is furnished with a single enormous horn-like tusk, and the Beluga, or White Whale (Delphinopterus), closely allied
1 See "A Review of the Family Delphinidæ," by Frederick W. True. Bull. U.S. Nat. Mus., No. 36 (Washington, U.S., 1889).