Page:The Zoologist, 4th series, vol 1 (1897).djvu/255

Rh whilst Phoca is common to it and Arctirenia; by the absence of Sirenians; and by the possession of three peculiar genera of Cetaceans, Hyperoodon, Delphinapterus, and Monodon.

II. sole possessor of the Monk Seal, Monachus, amongst the Pinnipeds, and of the Sirenian genus, Manatus.

III. characterized by the presence of the Sirenian Halicore and by the absence of Pinnipeds.

IV. 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. without true Seals, Phocinæ, but having Otaria and Macrorhinus from the south; no Sirenian known.

VI. characterized by four endemic genera of Phocidæ, and by the presence of many Otariæ; without Sirenians, but with two endemic forms of Cetaceans, Neobalæna, and Berardius.

In conclusion I will call attention 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 Straits, 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 I 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 and African coasts. The Manatee could hardly live to cross the Atlantic. It is only found close to the coast, where it browses on sea-weeds and other vegetable food in shallow water. How did it travel from America to Africa (or vice versa), unless there were a continuous shore-line between them? The same may be said of the Monk Seal, Monachus, of which one species