It seems desirable to open this subject with a brief discussion of the present distribution of living pelagic mammals, for without this introduction the relative importance of the various types for intercontinental correlation might not be clearly understood.
All the living and fossil pelagic mammals now known may be referred to three larger groups—the Sirenia, the Carnivora, and the Cetacea. Of these the sirenians, or sea-cows, are probably the most retiring, the least aggressive, and more local in their distribution. The pinnipeds, which are pelagic carnivores and are generally known as seals, walruses, and sea-lions, congregate in colonies at favorable localities, and at least one member of this group, the fur seal, makes long periodic migrations from its summer breeding places to its winter feeding grounds. Another sub‐order of the Carnivora has had at least one pelagic type, the sea-otter. The range of this animal formerly included the coasts . . .