1897.] DISTRIBtJTTOiS"' OY MARTJTE MAMMALS. 353 



their geographical distribution. Most oi: the tonus appear to be 

 very widely distributed, but it may be said geaerally that Dolphins 

 are most abundant in the inter-tropical seas and less plentiful both 

 to the north and south of them. 



There are, hoAvever, two forms that are exclusively inhabitants 

 of the Xorth Atlantic. These are the very remarkable Narwhal 

 (2Ionodon), in which the male is furnished with a single enormous 

 horn-like tusk, and the Beluga or Wliite Whale {BelpTxinapti'rus), 

 closely allied to the Narwhal in many points of its general structure. 

 These may be looked upon as quite isolated forms characteristic of 

 the Arctic portion of the Atlantic but not known in the Pacific \ 



V. Division of the Marine Area of the Globe into Sea-regions. 



From what has been already said, it will be evident that although 

 many of the Marine Mammals have a wide distriljution, others are 

 very definitely localized ; and a study of the latter will, I think, 

 enable us to divide the oceajiic portion of the globe iuto six Sea- 

 regions, corresponding to a certain extent with the six Land- 

 regions into which I proposed to separate the terrestrial portion 

 of the globe in 1874, and which were subsequently adopted by 

 Mr. Wallace in his standard work on the Geographical Distribution 

 of Animals. I propose to call these Sea-regions :■ — • 



(1) The North-Atlais'tic Sea-begiox, or Arctatlantis (Hoktos 

 and 'ArXav-(s=the daughter of Atlas), consisting of the northern 

 portion of the Atlantic down to about 40° N. lat. 



(2) The Mid-Atlantic Sea-eegion, or Mesatlaxtis (fxiaos and 

 ' AtXui'tIs), consisting of the middle portion of the Atlantic down 

 to about the Tropic of Capricorn. 



(3) The Indian Sea-region, or Indopelagia("Iicos and TreXayos), 

 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 (upK-os 

 and e(|o//j'J7=pax), containing the northern portion of the Pacific 

 Ocean down to about the Tropic of Cancer. 



(5) The Mid-Pacific Sea-region, or Mesirenia {jxiaoi and 

 e'lfiriiTj), containing the inter-tropical portion of the Pacific Ocean ; 

 and finally 



(6) The Southern Sea-region, or Notopelagia (roros and 

 ireXayos), 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 {Halichcerus) and the Bladder-Seal (C'^s^op/iora), are exclusively 



1 In a recent letter to ' Science' (1897, p. 843) Dr. Dall has pointed out 

 tliat this is an error. Both Mcnodon and DelpMna'pteriis occur in the North 

 racific— P.L.S., 4. vi. 97. 



