399 
Mr. Dillwyn on fossil shells. 
sioned the entire destruction of some genera, and a migration 
of others to the southward. 
It is highly probable, when a more perfect knowledge of 
the testaceous animals has been obtained, that the line of 
enquiry which I have now suggested may be greatly ex- 
tended, and the collected tendency of such analogies between 
the habits of living animals and the organic remains of the 
different strata, may serve to throw some light on the nature 
of the changes which the surface of our planet has under- 
gone. 
I am, my dear Sir, 
Yours very sincerely, 
L. W. Dillwyn. 
Penllergare, 
May 19, 1823. 
