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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 Balaenidae, usually divided into five genera: Balmna, Neobalmna, Rhachianectes, Megaptera, and Balmnoptera. Of these,
Balmna (Fig. 46, p. 204), Megaptera, and Balmnoptera 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 Neobalmna 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 Physeteridse 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 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 Pacific Ocean.
The second (existing) family of Toothed Whales contains only the Platanistidm, or Freshwater Dolphins, which although, in some cases, at the present day entirely fluviatile, must probably have descended from oceanic forms.1 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
1 Sir William Flower ("Whales, Past and Present," Proc. Roy. Insl., 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, Delphinidm, 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,1 both the genera and species of Delphinidm are still so imperfectly understood that not much can be said about their geographical distribution. Most of the forms
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 (Detyhinaptervs), closely allied
i See "A Review of the Family Delphinidae," by Frederick W. True. BuU. U.S. Nat. Mus., No. 36 (Washington, U.S., 1889).
to the Narwhal in many points of its general structure (Fig. 50). These may be looked upon as quite isolated
forms characteristic of the Arctic portion of the Atlantic and Pacific.
Section V.—Division Of The Marine Area Of The Globe Into Sea-regions
From what has been already said, it will be evident that although many of the marine mammals have a wide distribution, others are very definitely localized; and a study of the latter will enable us to divide the oceanic portion of the globe into six Sea-regions, corresponding to a certain extent with the six Land-regions already discussed. It is proposed to name these Sea-regions, which are shown in the map (Plate VIII., p. 216), as follows:—
(1) The North Atlantic Sea-region, or Arctatlantis (apKTos and 'atxclvtis = the daughter of Atlas), consisting of the northern portion of the Atlantic down to about 40° N. lat.
(2) The Mid-Atlantic Sea-region, or Mesatlantis (n€<ros and 'ArXavTk), consisting of the middle portion of the Atlantic down to about the Tropic of Capricorn.
(3) The Indian Sea-region, or Indopelagia ("Ii/So? and 71-6X070?), containing the Indian Ocean down to about the same degree of S. lat., and extending from the coast of Africa on the west to Australia and the great Oriental islands on the east.
(4) The North Pacific Sea-region, or Arctirenia (o/jkto? and elprjvr) = pax), containing the northern portion of the Pacific Ocean down to about the Tropic of Cancer.
(5) The Mid-Pacific Sea-region, or Mesirenia (fieaoi and elprfinf), containing the inter-tropical portion of the Pacific Ocean; and finally,
(6) The Southern Sea-region, or Notopelagia (voto<s and -n-eXayo^), containing the whole of the South Polar Ocean all round the globe south of the above-mentioned limits.
We will now proceed to consider shortly the characteristic mammals of these six Sea-regions.
Section VI.—The North Atlantic Sea-region,
Amongst the Pinnipeds two well-marked generic forms, the Grey Seal (Halichcerws) and the Bladder-Seal (Cystophora), are exclusively confined to Arctatlantis. The True Seals (Phoca) and the Walrus (Trichechus) are found in this region and in Arctirenia; and of the former genus three species (P. vitidina, P. grcenlandica, and P. barbata) are actually common to both these Sea-regions, while the Walruses (Trichechus rosmarus and T. obesus) of the two Sea-regions are perhaps somewhat doubtfully distinguishable. It may be easily understood how this has come to pass,