222 THE ZOOLOGIST. 



and Eif>Yiw = pax), containing the northern portion of the Pacific 

 Ocean down to about the Tropic of Cancer. 



(5) The Mid-Pacific Sea-region or Mesirenia (^os- and 

 tlprivv), containing the inter-tropical portion of the Pacific Ocean ; 

 and finally 



(0) The Southern Sea-region or Notopelagia (vqtos and 

 TreXayor), 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. 



VI. The North Atlantic Sea-region, or Arctatlantis. 

 Amongst the Pinnipeds two well-marked generic forms, the 

 Grey Seal, Halichoerus, 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. grcenlandica, and P. barbata—&re actually common to both 

 these sea-regions, while the Walruses — Trichechus rosmarus and 

 T. obesus — of the two sea-regions are perhaps somewhat doubt- 

 fully 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 (seeP.Z.S. 1882, p. 311) 

 a " lipotype " of Arctatlantis, but what I now propose to designate 

 a " lipomorph."* 



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

 group which characterises a particular locality by 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 characterises a particular country may be called a " topomorph " 

 (roTrog, locus, and poppy, forma). Thus, in Africa Giraffa and Phacoch cents 

 would be " topomorphs," and Cervus and Ursus would be " lipomorphs." 



