DISTRIBUTION OF MARINE MAMMALIA. 223 



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 repre- 

 sented in the North Atlantic by Balcena, Megaptera, and Balce- 

 noptera, but of these the two latter are almost universally 

 distributed over the ocean, and Balcena 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 Balcena mysticetus has never been found else- 

 where. 



Proceeding to the Odontoceti, the case is different. Amongst 

 the Physeteridce, Hyperoodon is confined to Arctatlantis, and, as 

 already explained, two very well-marked types of the Delphinidce, 

 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. Halichoerus, Cysto- 

 phora, Hyperoodon, Delphinapterus, and Monodon, 



VII. The Middle Atlantic Sea-region, or Mesatlantis. 



Mesatlantis has certainly not so many forms of marine mam- 

 mals 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, Phocince, 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 well- 

 marked Sirenian genus Manatus, one species of which, M. ameri- 

 canus, frequents the coast of America, and another, M. senegalensis, 

 that of Africa. 



As regards the Cetaceans, we are not able to say that Mes- 

 atlantis, although well furnished with many generic types of this 

 order, has any one peculiar to it. We must therefore rest con- 

 tent with assigning two genera of marine mammals, Monachus 

 and Manatus, as characteristic forms or topomorphs of the sea- 

 mammal-life of Mesatlantis. 



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