GEOLOGICAL DISTRIBUTION 107 
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. Cartnozoic on TERTIARY— 
1. Pleistocene—River alluvia, ete. 
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, ete. 
III. Patzozorc on Primary— 
. Permian—Beds overlying the Coal. 
. Carboniferous—Coal-measures, ete. 
. Devonian—Old Red Sandstone. 
. Silurian— Wenlock Limestone, ete. 
. Cambrian—Llanberis Slate, ete. 
. Archean—Gneiss and other schists. 
Qa Pk whe 
The names in the first column indicate the primary divisions or 
life-periods, while those in the second column are the great systems, 
each of which is again divided into minor groups, the popular 
names of a few of these minor groups being given in the third 
column. There are at present no means of arriving at any satis- 
factory conclusion as to the absolute length of time indicated by 
