354 ^iR. P- L- scLATBE ON THE [Mar. 1 6, 



confined to Arctatlantis. The True Seals {Phoca) and the Walrus 

 {Tricliechus) are found in this region and in Arctirenia ; and of 

 the former genus three species (P. vitnlina, P. f/r(eidandica, and 

 P. harhata) are actually common to both these Sea-regions, while 

 the Walruses (IVichcchus rosnmrus and T. ohcsus) of the two Sea- 

 regions are perhaps somewhat doiibtfully 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 Arctatlautis 

 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 (see P. Z. S. 

 1882, p. 311) a " lipotype " 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 Mystacoceti well represented 

 in the North Atlantic by Balcena, Megaptera, and BaJcmoptera ; but 

 of these the two latter are almost universally distributed over the 

 ocean, and Balcena recurs again in the Xorth Pacific as well as in 

 more southern latitudes, so that there is no genus of Whalebone 

 Whales peculiar to Arctatlantis. 



Proceeding to the Odontoceti, the case is different. Amongst the 

 PJiysetei-idfe, //y^xjcoof^ou is confined to Arctatlantis, and, as already 

 explained, two very well-marked types of the Ddph'niid<e, Delphin- 

 apterus and Moiwdon, are likewise exclusively denizens of the 

 Xorth Atlantic Ocean. Arctatlantis therefore may be said to be 

 well characterized by the possession of at least five genera of Marioe 

 Mammals not found elsewhere, viz. Ha?ichm-us, Oi/sto^diora, Hy- 

 peroodon, Delphi napterns, and Monodon. 



VII. Tlie Mid-Atlantic Sea-region^ or Mesatlaiitis. 



Mesatlantis has certainly not so many forms of Marine Mam- 

 mals confined to its area as Arctatlantis, but there seem to be good 

 oTounds for its separation. As we descend towards the tropics the 

 true Seals (Phocina'), 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 Moaachus or Monk- 

 Seal restricted to Mesatlantis, one species (M. aJhiventer) occurring 



1 On formei' occasions I have used the term " lii)otype " for a natui-cal group 

 which cliaracterizes a particular locality by its absence. It would, however, 

 perhaps be better to change the term to "lipomorph," because the term "type" 

 and its compounds have been generally employed in reference to the particular 

 specimens of a species upon which original descriptions have been based 

 (t/. Thomas, P, Z. S. 1893, p. 241). In the same way a natural group which 

 characterizes a particular country may be called a "topomorph" (roiros, loctis, 

 and iiop<pi], forma). Thus in Africa Giraffa and Phacochxrus would be " topo- 

 morphs," and Cerims and Ursus would be " lipomorphs." 



