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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, Balæna, Megaptera, and Balænoptera are all represented in the North Pacific, and also many species of Delphinidæ 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,
OR MESIRENIA 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, Balænoptera, Physeter, Cogia, and Ziphius certainly occur there, besides many representatives of the widely spread Delphinidæ.
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 Phocidæ — 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 Otariæ 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 (Balæna), Notopelagia has a smaller representative of the group (Neobalæna) entirely restricted to its area. It has also representatives of Megaptera and Balenoptera, 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 (Delphinidæ) 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 (Phocine), of which two genera, Halichorus 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 MonkSeals, Monachus, amongst the Pinnipeds, and of the Sirenian genus Manatus.
III. Regio Indopelagica, characterized by the presence of the Sirenian Halicore and by the absence of Pinnipeds.
IV. Regio Arctirenica, with Phoca like the Regio Arctatlantica, but having Otaria also; the home of the (now extinct) Sirenian Rhytina and of the endemic Cetacean Rhachianectes.
V. Regio Mesirenica, without true Seals (Phocinæ), but having Otaria and Macrorhinus from the south; no Sirenian being known there.
VI. Regio Notopelagica, characterized by four endemic genera of Phocidæ, and by the presence of many Otarix ; without Sirenians, but with two endemic forms of Cetaceans (Neobalena and Berardius).
In conclusion, attention may be called to some of the more remarkable points in the general distribution of the marine mammals and to their apparent significance.
In the first place it is evident that the Pacific has much more in common with the Notopelagian Region than the Atlantic. Otaria and Macrorhinus, quite unknown in the Atlantic, extend themselves to the northern extremity of the Pacific, the former pervading that ocean up to Behring's Strait, and the latter reaching to the Californian coast. It follows that in former ages there must have been some barrier in the Atlantic which did not exist in the Pacific to stop their progress northwards. The only barrier one can imagine that would have effected this must have been a land uniting South America and Africa, across which they could not travel. Adopting this hypothesis, we have at the same time an explanation of the presence of the Manatee on both the American