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 Balæna, Megaptera, and Balænoptera, 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, although the great Balæna mysticetus has never been found elsewhere.
Proceeding to the Odontoceti, the case is different. Amongst the Physeteridæ, Hyperoodon is confined to Arctatlantis, and, as already explained, two very well-marked types of the Delphinidæ, Delphinapterus and Monodon, are likewise exclusively denizens of the North Atlantic Ocean. Arctatlantis therefore may be said to be well characterized by the possession of at least five genera of marine mammals not found elsewhere, viz. Halichœrus, Cystophora, Hyperoodon, Delphinapterus, and Monodon.
VII. The Middle Atlantic Sea-region, or Mesatlantis.
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, Phocinæ, 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 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.
