May 14, 1897.] 



SCIENCE. 



745 



vitulini, P. groemlandica, and P. barbata) 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 dis- 

 tinguishable from Arctatlantis, by the pres- 

 ence of Eared Seals (Otaria), which is 

 utterly unknown in the whole of the At- 

 lantic area. Otaria is, in fact, as regards 

 Arctatlantis what I have called, on previous 

 occasions (see P. Z. S. 1882, p. 311), a ' lip- 

 otype ' of Arctatlantis, but what I now pro- 

 pose 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 Mysta- 

 coceti well represented in the North Atlantic 

 by Balcena, Mega-ptera and Bolcenoptera, but 

 of these the two latter are almost univer- 

 sally distributed over the ocean, and Balmna 

 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 Balcena 

 mysticetus has never been found elsewhere. 



Proceeding to the Odontoceti, the case is 

 different. Amongst the PhyseteridcB, Hy- 



* On former occasions I have used the term ' lipo- 

 type ' for a natural group which characterizes a par- 

 ticular locality by its absence. It would, however, 

 perhaps be better to change the term to ' lipomorph,' 

 because the type and its compounds have been gener- 

 ally employed in reference to the particular speci- 

 mens of a species upon which original descriptions 

 are based (c/. Thomas, P. Z. S. 1893, p. 241). In the 

 same way a natural group which characterizes a par- 

 ticular country may be called a 'topomorph' (roirof 

 locus and /iopcpr/ forma ) . Thus in Africa Giraffa and 

 Phacochcenis would be ' topomorphs, ' and Cervus 

 and Ursus would be ' lipomorphs. ' 



peroodoii is confined to Arctatlantis and, as 

 already explained, two very well-marked 

 types of the Delphinidte, Delphinapterus and 

 Monodon, are likewise exclusively denizens 

 of the North Atlantic ocean. Arctatlantis? 

 therefore, may be said to be well character- 

 ized by the possession of at least five genera 

 of Marine Mammals not found elsewhere, 

 viz., Salichcerus, Cystophora, Syperoodon, Del- 

 phinapterus, and 3£onodon. 



VII. THE MIDDLE ATLANTIC SEA-EEGION, OR 



MESATLANTIS. 



Mesatlantis has certainly not so many 

 forms of Marine Mammals confined to its 

 area as Arctatlantis, but there seem to be 

 good grounds for its separation As we 

 descend towards the tropics the true Seals 

 (Phocince), which are constituted to live in 

 colder water, gradually fall off in number, 

 and in Mesatlantis are no longer met with. 

 But in their place we find the genus Mona- 

 chus or Monk Seal restricted to Mesatlantis, 

 one species (il/. albiventer') occurring in the 

 Mediterranean and on the North African 

 coast, and a second (ilf. tropicalis) being 

 found in the West Indies. Mesatlantis is 

 likewise the true home of the well-marked 

 Sirenian genus Manatus, one species of which 

 (if. americanus) frequents the coast of 

 America and another (if. senegalensis) that 

 of Africa. 



As regards the Cetaceans we are not able 

 to say that Mesatlantus, although well- 

 furnished with many generic types of this 

 Order, has one peculiar to it. We must, 

 therefore, rest content with assigning two 

 genera of Marine Mammals, Monoehus and 

 Manatus, as characteristic forms or topo- 

 morphs of the Sea-Mammal life of Mesat- 

 lantis. 



VIII. THE INDIAN SEA-EEGION, OR INDOPE- 



LAGIA. 



The Marine Carnivora, so far as we know, 

 are entirely foreign to Indopelagia, but the 



