DISTRIBUTION OF MARINE MAMMALIA. 221 



knowledge of them,* both the genera and species of Belphinidce 

 are still so imperfectly understood that I cannot say much about 

 their geographical distribution. Most of the forms appear to be 

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

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

 both to the north and south of them. 



There are, however, two forms that are exclusively inhabitants 

 of the North Atlantic. These are the very remarkable Narwhal, 

 Monodon, in which the male is furnished with a single enormous 

 horn-like tusk, and the Beluga or White Whale, Defohinapteras, 

 closely allied to the Narwhal in many points of its general struc- 

 ture. These may be looked upc " .3 quite isolated forms charac- 

 teristic 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 distribution, 

 others are very definitely localized ; and a study of the latter will, 

 I think, enable us to divide the oceanic portion of the globe into 

 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 Atlantic Sea-region or Arctatlantis (upKTo; 

 and oh-Aavr/s^the daughter of Atlas), consisting of the northern 

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



(2) The Mid-Atlantic Sea-region or Mesatlantis (jaeo-os 

 and aTAavr/s), consisting of the middle portion of the Atlantic 

 down to about the Tropic of Capricorn. 



(3) The Indian Sea-region or Indopelagia (iv$k and irsxayos), 

 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 (apxro; 



* See " A Review of the Family Delphinidce^ by Frederick W. True : 

 Bull. U.S. Nat. Mus. No. 36, Washington, U.S. 1889. 



Zool. 4th ser. vol. L, May, 1897. E 



