268 Notices of Memoirs — Dr. P. L. Sclater — 



part of the ocean. Mesoplodon is also widely distributed, but is 

 apparently more abundant in the Southern Hemisphere, Berardiiis, 

 however, so far as we know at present, is restricted to the South 

 Polar Ocean. 



The third family of Toothed Whales contains only the Platanistidae, 

 or Fresh-water Dolphins, which, although, in some cases, at the 

 present day entirely fluviatile, must probably have descended from 

 oceanic forms. The three known genera are Platanista of the 

 Ganges and Indus, Inia of the river Amazons, and Pontoporia of 

 tlie river La Plata ; the last form making a connecting link between 

 the two preceding genera and the marine Dolphins. 



The fourth family of Toothed Whales, containing the Dolphins, 

 Delphinidas, is very numerous in species and embraces at least fifteen 

 or sixteen genera. But in spite of the efforts of Mr. True, who 

 has recently given us an excellent summary of our present know- 

 ledge of them,^ both the genera and species of Delphinidae 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 genera] ly that Dolphins are 

 most abundant in the intertropical 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 vei-y remarkable Narwhal 

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

 horn-like tusk, and the Beluga or White- Whale (Delphinapterus) , 

 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 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 standai'd work on the Geographical Distribution of Animals. I 

 propose to call these Sea-regions — 



(1) The North Atlantic Sea-region or Arctatlantis (upKros and 

 'ATXaj/T/9=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 (neao9 and 

 'AtXoj/t/?), consisting of the middle portion of the Atlantic down to 

 about the Tropic of Capricorn. 



^ See "A Review of the family Delphiuidffi," by Frederick W. True: Bull. 

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



■•^ Presidential Address, Sect. Biology, Brit. Assoc. 1875 (Bristol) ; and Proc. Roy. 

 Inst., vol. viii, pp. 611-13 (1878). 



