Distribution of Marine Mammalia. 269 



(3) The Indian Sea-region or Indopelagia ( 'Ij/oo? and TreXarfos), 

 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 (upi:T09 and elp/jvri 

 =:pax), containing the northern portion of the Pacific Ocean down 

 to about the Tropic of Cancer. 



(5) The Mid-Pacific Sea-region or Mesirenia {/Lieao? and eipi'jm]), 

 containing the intertropical portion of the Pacific Ocean ; and finally, 



(6) The Southern Sea-region or Notopelagia (j/o'to? and 7re\ar^o9), 

 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. 



YI. The North Atlantic Sea-region, or Arctatlantis. — Amongst the 

 Pinnipeds two well-marked generic forms, the Grrey Seal (^HalichcBrus) 

 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. Groenlandica, and P. barbafa) 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 may in 

 the course of time, during unusually mild summers, 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 what I have 

 called on previous occasions (see P.Z.S. 1882, p. 311) a " lipotype " 

 of Arctatlantis, but what I now propose to designate 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 Balcena, Megnptera, and Balcenoptera, 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 Balcsna mysticetus 

 has never been found elsewhere. 



Proceeding to the Odontoceti, the case is different. Amongst the 



1 On former occasions I have used the term "lipotype" for a natural group 

 ■whicli characterizes a particular locality hy its absence. It would, however, 

 perhaps be better to change the term to "lipomorph," because the type and its 

 compounds have been generally employed in reference to the particular specimens of 

 a species upon which original descriptions are based (cf. Thomas, P.Z.S. 1893, 

 p. 241). In the same way a natural group which characterizes a particular country 

 may be called a " topomorph " {t6itos, locus, and fxopfpij, forma). Thus in Africa 

 Giraffa and Phacochonrus would be " topomorphs," and Cervus and Zfrsus would be 

 " lipomorphs." 



