The Pinnipeds, although capable of traversing long reaches of ocean, are less truly aquatic than the last two groups, always resorting to the land or to extensive ice-floes for the purpose of breeding. The geographical range of the various species is generally more or less restricted, usually according to climate, as they are mostly inhabitants either of the Arctic or Antarctic seas and adjacent temperate regions, very few being found within the tropics. For this reason the northern and the southern species are for the most part quite distinct. In fact, the only known exception is the case of a colony of the Sea-Elephant (_Macrorhinus leoninus_), the general range of which is in the southern hemisphere, inhabiting the coast of California. Even in this case a different specific name has been given to the northern form; but the characters by which it is distinguished are not of great importance, and probably, except for the abnormal geographical distribution, would never have been noticed. The most remarkable circumstance connected with the distribution of the Pinnipeds is the presence of members of the suborder in the three isolated great lakes or inland seas of Central Asia—the Caspian, Aral, and Baikal; these forms, notwithstanding their long isolation, having varied but slightly from species now inhabiting the Polar Seas. II. GEOLOGICAL DISTRIBUTION. _Geological Sequence._—In order to understand the geological distribution, or in other words the distribution in time of mammals, it is necessary to be acquainted with the chief divisions, or time-periods, of the strata constituting the crust of the globe. These are shown in the following table, which commences with the uppermost or most recent beds and ends with the lowest and oldest. I. CAINOZOIC OR TERTIARY— 1. Pleistocene—River alluvia, etc. 2. Pliocene—Suffolk Crag. 3. Miocene—Hempstead Beds of Hampshire. 4. Eocene—Paris Gypsum and London Clay. II. MESOZOIC OR SECONDARY— 1. Cretaceous—Chalk, Greensands, etc. 2. Jurassic—Oolites and Lias. 3. Triassic—Red Marls, Dolomites, etc. III. PALÆOZOIC OR PRIMARY— 1. Permian—Beds overlying the Coal. 2. Carboniferous—Coal-measures, etc. 3. Devonian—Old Red Sandstone. 4. Silurian—Wenlock Limestone, etc. 5. Cambrian—Llanberis Slate, etc. 6. Archæan—Gneiss and other schists.