OCR Text |
Show 354 MR. p. L. SCLATER O N T H E [Mar. 16, confined to Arctatlantis. The True Seals (Phoca) and the Walrus (Trichechus) are found in this region and in Arctirenia; and of the former genus three species (P. vitullna, P. grcenlandica, and P. harhata) 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, dining 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 Arctatlantis 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 propose to designate a " lipomorph." l The Sirenians are entirely absent from the North Atlantic and constitute another lipomorph of that area. Coming to the Whales, we find the Mgstacocetl well represented in the North Atlantic by Balcena, Megaptera, and Balcenoptera ; but of these the two latter are almost universally distributed over the ocean, and Balcena 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. Proceeding to the Odontoceti, the case is different. Amongst the Physeteridce, Hyperoodon is confined to Arctatlantis, and, as already explaiued, two very well-marked types of the Delphinidcs, Delphinaptcrus and Monodon, are likewise exclusively denizens of the North Atlantic Ocean. Arctatlantis therefore may be said to be well characterized by the possession of at least five genera of Marine Mammals not found elsewhere, viz. Halichcerus, Cystophora, Hyperoodon, Delphiilapterus, and Monodon. VII. The Mid-Atlantic Sea-region, 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 (Phoclnce), 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 Monachus or Monk- Seal restricted to Mesatlantis, one species (M. albiventer) occurring 1 On former occasions I have used the term " lipotype " for a natural group which characterizes a particular locality by its absence. It would, however, perhaps be better to change tbe 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 (of. Thomas, P, Z. S. 1893, p. 241). In the same way a natural group which characterizes a particular country may be called a "topomorph" (TOKOS, locus, and fiopipr), forma). Thus in Africa Giraffa and Pkacpcharus would be " topo-morphs," and Cervus and Ursus would be " lipomorphs." |