Notices of Memoirs — Dr. Sclafer — Marine Mammalia. 265 



The obliquity of these suborbicular valves (about 60°) approxi- 

 mates to that of Cyclestheria, as defined by G. 0. Sars ^ ; but the 

 latter is much more nearly circular in outline (Fig. 6). 



The more protuberant umbo, further from the anterior corner, 

 and the more distinctly level straightness of the hinge-line behind 

 it, are distinctive characteristics, as well as the somewhat more 

 prolate antero-dorsal region of the valve in front of the umbo. 



{To be concluded in our next Number.) 



isTOTioiES OIF nvnzEnvnoiias- 



I. — On the Distribution of Marine Mammals.- By P. L. Sclater, 



M.A., Ph.D., F.R.S., Sec. Zool. Soc. 



(Read before the Zoological Society, March 16, 1897. 



I. Introductory BemarTcs. — Most of the recent writers on Geo- 

 graphical Distribution have confined their attention to terrestrial 

 mammals, or at any rate have but casually alluded to the marine 

 groups of that class. On the present occasion I wish to call your 

 attention to some of the principal facts connected with the dis- 

 tribution over the world's surface of the marine or aquatic members 

 of the Class of Mammals. 



Aquatic mammals, which pass their lives entirely, or, for the 

 greater part, in the water, are, of course, subject to very different 

 laws of distribution from those of the terrestrial forms. As regards 

 aquatic mammals, land is of course an impassable barrier to their 

 extension, and, subject to restrictions in certain cases, water 

 offers them a free passage. Just the opposite is the case with the 

 terrestrial mammals, to which in most cases land offers a free 

 passage, while seas and rivers restrain the extension of their ranges. 



The groups of aquatic mammals that are represented on the 

 earth's surface at the present time are three in number, viz. : (1) 

 the suborder of the Carnivora, containing the Seals and their allies, 

 generally called the Pinnipedia, which are semi-aquatic ; (2) the 

 Sirenia, which are mainly aquatic ; and (3) the Cetacea, which 

 never leave the water, and are wholly aquatic. We will consider 

 briefly the principal representatives of these three groups, following 

 nearly the arrangement of them employed in Flower and Lydekker's 

 "Mammals, living and extinct." 



1 Christiania Vidensk.-Selsk. Forhandl. 1887, No. 1, pi. i, figs. 1-3. Dr. Baird 

 gave a rather more symmetrical form to this subcircular species (Proc. Zool. Soc. 

 1859, p. 232 ; and 1860, p. 188, pi. Ixiii, fig. 1), triangular on the dorsal and 

 rounded on the ventral margin. His figure, if placed with the hinge-line horizontal, 

 shows a subceutral umbo, with the hinge-line almost level behind it, and equal to 

 half of the transverse diameter of the valve. There is a rapid slope in front, 

 meeting the anterior part of the more than semicircular free margin. 



^ For the Land-Provinces proposed by the author for the Geographical Distribution 

 of Birds, see Dr. P. L. Sclater's paper in Journ. Proc. Linn. Soc. (Zoology) 1858, 

 vol. ii, pp. 130-45; also Eep. Brit. Assoc. 1860, Trans. Sec, pp. 121, 122; 

 H. Woodward, Geol. Mag. 1885, Decade III, Vol. II, p. 315, and Proc. Geol. 

 Assoc. 1886, vol. ix, No. 5, p. 366. 



