302 MOLLUSCA FROM THE CRAG. 



Marine Fauna of the entire deposit, or may we be permitted to select a part only for 

 such examination, and if so, what part ? 



The Mollusca herein described bear so close a resemblance to animals now living in 

 our own seas, as to give good reason to believe their geological relationship to be much 

 nearer to the present Period than to the Eocene ; and if an amount of extinction of 

 more than one half of its species be necessary to entitle a Deposit to be considered as 

 belonging to what is called the Miocene or Middle Tertiary, our present identifications 

 do not fulfil those required conditions, even for the lowest or oldest (by position) of the 

 Crag Formations. 



Assuming that a different construction might be put upon a few of the specific 

 determinations, in opposition to the conclusions I have arrived at, I much doubt 

 whether the Coralline Crag could possibly be made to contain more than 50 per cent, 

 of extinct species of Mollusca ; while the connection zoologically between this Deposit 

 and those of the Eocene is so small as to have an identity of less than 1 per cent, that 

 have transmitted their posterity unaltered from those Periods into the Crag; and 

 although a considerable difference of conditions probably existed under which the 

 Formations were deposited, Tropical forms are by no means wholly excluded from 

 the Coralline Crag Sea, neither are sub- Arctic genera, such as Glycimeris, Astarte, and 

 Cyprina, absent from the Older Tertiaries. 



When the present work was begun, I had purposed to call it simply ' A Monograph 

 of the Crag Mollusca ;' but this title had to be submitted to the Council of the 

 Palseontographical Society for their approval, when the term " Crag " was thought by 

 some of the members of that body to be of too local or technical a significance, and 

 would not be fully understood by foreign geologists ; and the explanatory addition of 

 ' Descriptions of Shells from the Middle and Upper Tertiaries of England ' was then 

 suggested, and acceded to by myself. 



A more complete examination of these Deposits, during the progress of the work, 

 has induced me to believe the term " Middle" to have been incorrectly introduced, 

 there being no remains of a Formation in Great Britain referable to that Period, more 

 especially if we are to depend, for such determination, upon the amount of extinction 

 by the per ccntage mode of valuation ; it is therefore requested to erase the words 

 Middle and Miocene from the title-pages and other parts of the work formerly given, 

 as 1 believe the Formations I have been here attempting to illustrate belong with more 

 propriety to the Upper Tertiaries. 



