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,1 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. (Delphinus delphis.) [Flower and Lyd. Mamm., p. 271.] 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 (Delphinapterus), 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). to the Narwhal in many points of its general structure (Fig. 50). These may be looked upon as quite isolated FIG. 50.-THE BELUGA. (Delphinapterus leucas.) [Flower and Lyd. Mamm., p. 262.] 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 (ἄρκτος and 'Ατλαντίς = 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 (μéσos and 'ATλavris), consisting of the middle portion of the Atlantic down to about the Tropic of Capricorn. (3) The Indian Sea-region, or Indopelagia ("Ivdos and wéλayos), 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 (ἄρκτος and εἰρήνη = pax), containing the northern portion of the Pacific Ocean down to about the Tropic of Cancer. (5) The Mid-Pacific Sea-region, or Mesirenia (μéoos and eipývn), containing the inter-tropical portion of the Pacific Ocean; and finally, (6) The Southern Sea-region, or Notopelagia (vótos and Téλayos), 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, OR ARCTATLANTIS Amongst the Pinnipeds two well-marked generic forms, the Grey Seal (Halichorus) 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. vitulina, P. grænlandica, 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, because the Seals and Walrus in the course of time, during unusually mild summers, may have extended themselves along the north coast of the American continent into the Northern Pacific. But Arctirenia, as we shall presently show, is markedly distinguishable from Arctatlantis by the presence of Eared Seals (Otaria), which are utterly unknown in the whole of the Atlantic area. Otaria is in fact, as regards Arctatlantis, a lipomorph. The Sirenians are entirely absent from the North Atlantic and constitute another lipomorph of that area. Coming to the Whales, we find the Mystacoceti well represented in the North Atlantic by Balana, Megaptera, and Balaenoptera; but of these the two latter are almost universally distributed over the ocean, and Balæna recurs again in the North Pacific as well as in more southern latitudes, so that there is no genus of Whalebone Whales peculiar to Arctatlantis. Proceeding to the Odontoceti, the case is different. Amongst the Physeterida, Hyperoodon is confined to Arctatlantis. Arctatlantis therefore may be said to be well characterized by the possession of at least three genera of marine mammals not found elsewhere, viz., Halichorus, Cystophora, and Hyperoodon, and by the complete absence of the Eared Seals (Otariida). SECTION VII.-THE MID-ATLANTIC SEA-REGION, Mesatlantis has certainly not so many forms of marine mammals confined to its area as Arctatlantis, but there seem to be good grounds for its separation. As we descend towards the tropics the true Seals (Phocina), 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 Monachus, 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, Monachus 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 |