64 BRITISH AND EUROPEAN FOSSIL MASTODONS. 



from the Eocene period upwards, are inferred to have been 

 deposited in the reconstructed strata also within the Pliocene 

 period, since M. (Tetraloph.) Arvemensis, which occurs so 

 abundantly in the Red Crag, has not been met with anywhere 

 on the Continent of Europe except in deposits of a Pliocene 

 age. The Red Crag sea appears to have breached a previously 

 established and populated Pliocene land, and to have buried 

 the bones referable to various epochs in the same sea- 

 bottom. 



In the preceding remarks I have purposely excluded any 

 reference to the Shell-evidence, and confined the comparison 

 strictly to the Mammalian Fauna. The Mollusca have 

 unquestionably been wielded as a most powerful exponent of 

 geological chronology, and of the successive physical changes 

 which have taken place on the surface of the earth. But it 

 will hardly be denied that the evidence presented by Mamma- 

 lian remains, when obtained in sufficient variety and abundance, 

 is of greater significance as a test of contemporaneous forma- 

 tion in geology, or the reverse : — 1st, Because Mammalian 

 genera and species are everywhere shown to be of more 

 limited duration in time than the Mollusca; and, 2ndly, 

 because from the vastly greater complexity of their relative 

 functions, they are much more susceptible of being affected 

 by the altered climatal conditions which are necessarily 

 involved in every great physical change, and which conduce 

 most to the extinction of species. 



The conclusions to which the comparison has led are : — 



1. That the Mastodon remains which have been met with 

 in the ' Pluvio-marine Crag' and ' Red Crag ' belong to a 

 Pliocene form, Mastodon (Tetralophodon) Arvemensis. 



2. That the Mammalian Fauna of the Fluvio-marine Crag 

 bears all the characters of a Pliocene age, and is identical 

 with the Sub-Apennine Pliocene Fauna of Italy. 



3. That the Red and Fluvio-marine Crags, tested by their 

 Mammalian Fauna, must be considered as beds of the same 

 geological age. 



