GEOLOGICAL DISTRIBUTION 107 



quite distinct In fact, the only known exception is the case of a 

 colony of the Sea-Elephant (Macrorhinus leoninvs), 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. Palaeozoic 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. Archaean — Gneiss and other schists. 



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 



