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 /ess than 1 per cent. that 
have transmitted their posterity wnaltered 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 
Paleeontographical 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 centage 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 I believe the Formations I have been here attempting to illustrate belong with more 
propriety to the Upper Tertiaries. 
