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towards the tropics the true Seals (Phocinm), which are constituted to live in colder water, gradually fall off in number, and in Mesatlantis are no longer met with. But in their place we find the genus Monaehus, or Monk-Seal, restricted to Mesatlantis, one species (M. albiventer) occurring in the Mediterranean and on the North African coast, and a second (M. tropicalis) being found in the West Indies. Mesatlantis is likewise the true home of the wellmarked Sirenian genus Manatus, one species of which (M. americanus) frequents the coast of America and another (M. senegalensis) that of Africa.
As regards the Cetaceans, we are not at present able to say that Mesatlantis, although well furnished with many generic types of this Order, has any one peculiar to it. We must therefore rest content with assigning two genera of marine mammals, Monaehus and Manatus, as characteristic forms or topomorphs of the Sea-mammal-life of Mesatlantis.
Section VIII.—The Indian Sea-region,
The marine Carnivora, so far as we know, are entirely foreign to Indopelagia, but the Sirenians are well represented by the Dugong (Halicore), which pervades all its northern coasts from North Australia to India and the Red Sea and down the African coast to the confines of British East Africa. Whether the species of Halicore found at different points within this area are the same or different is still a matter of discussion, but there can be no doubt that Halicore is an exclusive inhabitant of
Indopelagia. As regards the Whales of Indopelagia, we know that Physeter, Cogia, and Ziphius, and numerous forms of Delphinidm occur there, but are not aware of any Cetacean that is entirely restricted to this Sea-region.
Section IX.—The North Pacific Sea-region,
As was pointed out when speaking of Arctatlantis, Arctirenia has one genus of Phocidm (Phoca) in common with the North Atlantic, and three of the species of this genus appear to be actually identical in these two Searegions, whilst a fourth Phoca (P. fasciata) is only found in the North Pacific. The Walrus (Trichechus) is again a form of marine mammals common to both the great northern Sea-regions. But the feature of Pinnipedian life that absolutely distinguishes Arctirenia from Arctatlantis is the presence in the former of three (if not four) wellmarked species of the Eared Seals (Otariidm), which are absolutely unknown in the vast extent of the Atlantic down at least to 30° S. lat.
Arctirenia has unfortunately lost its Sirenian, Steller's Sea-cow (Rhytina stelleri), the largest modern representative of this formerly prevalent group, which since the days of the Pleistocene has greatly diminished in numbers, but I think we may still treat Rhytina as one of the characteristic forms of the Arctirenian Sea-region. The North Pacific is also, even at the present day, the sole possessor of a remarkable genus of Whalebone Whales which combines the long head and elongate form of BaUeonptera with the absence of the dorsal fin of BdUena. This is the Grey Whale, Rhachianectes glaucus of Cope, which, in these days, is confined to the North Pacific, and does not range farther south than the 20th parallel in that ocean. At the same time it should be stated that indications have been discovered that a nearly allied form existed in the Atlantic in previous geological ages, though this is by no means certain. Besides Rhachianectes, Balmna, Megaptera, and Balmnoptera are all represented in the North Pacific, and also many species of Delphinidm of which little is at present known. But Rhytina and Rhachianectes are the only genera of Marine Mammals absolutely confined to Arctirenia.
Section X.—The Mid-pacific Sea-region,
The Eared Seals, Otaria, must have necessarily passed through Mesirenia in their passage from south to north, though the only record of their actual presence in the central part of the Pacific is, so far as we know, the recent discovery of them in the Galapagos. It should be stated, however, that Tschudi records the occurrence of two species of Otaria on the islands of the coast of Peru, and that in 1802 Humboldt met with an Eared Seal on the Island of San Lorenzo, in the Bay of Callao, which is only some 12° south of the Equator.
Like Otaria, the Sea-elephant (Macrorhinus) has apparently in former ages travelled up the South American shores and established itself as far north on the coast of California as about 34° N. lat. The Californian Seaelephant has been discriminated by Gill as a distinct species (Macrorhinus angustirostris), but its differences from the southern form (M. leoninus) seem to be but trifling.
As regards the Cetaceans of Mesirenia, our information is at present very imperfect, and there is little to say except that species of Megaptera, Balmnoptera, Physeter, Cogia, and Ziphius certainly occur there, besides many representatives of the widely spread Delphinidm.
Section XI.—The Southern Sea-region, Or
The wide ocean which surrounds the Southern Pole on every side, and extends up to 40° S. lat., seems to present, as regards its marine mammals, a nearly homogeneous fauna, which we will now briefly consider. In the first place it contains representatives of four genera of true Phocidm — Ogmorhinus, Lobodon, Leptonychotes, and Ommatophoca, which are peculiar to the southern seas, and are quite distinct from all their northern representatives in the Arctic Ocean. The Sea-elephant, Macrorhinus, is also a denizen of Notopelagia, though, as we have already seen, it has wandered north along the South American coast far into Mesirenia.
Like Macrorhinus, Otaria also, containing the group of Eared Seals, appears to have been originally an Antarctic group, and the greater number of its species are still found in the Southern Ocean. But the Otarim have travelled still further north than Macrorhinus, and three, if not four, species are, as already stated, in these days wellestablished inhabitants of Arctirenia.
The Sirenians are absent from Notopelagia, but Cetaceans of every kind are abundant. Besides one or more representatives of the true Whalebone Whale (Balmna), Notopelagia has a smaller representative of the group (Neobalmna) entirely restricted to its area. It has also representatives of Megaptera and Balmnoptera, though it is doubtful how far they are even specifically distinct from some of their northern representatives.
Among the Toothed Whales (Odontoceti) we find a large Ziphioid form, Berardius, restricted to the Pacific area, while Ziphius and Mesoplodon also occur there. The Dolphins (Delphinidm) are likewise numerous, and present some distinct species, but not, so far as our present knowledge extends, any generic forms that do not occur elsewhere.
But Notopelagia is sufficiently distinguished from all the five more northern Sea-regions by possessing four genera of Seals and two of Cetaceans entirely restricted to its area.
It has therefore been shown that, for the geography of marine mammals, the ocean may be conveniently divided into six Sea-regions (Plate VIII., p. 216), which are as follows :—
I. Regio Arctatlantica, characterized by its Seals (Phocinm), of which two genera, Halichcerus and Cystophora, are peculiar, whilst Phoca is common to it and Arctirenia; by the absence of Sirenians; and by the possession of a peculiar genus of Cetaceans (Hyperoodon).
II. Regio Mesatlantica, sole possessor of the Monk