L 41 3 ] 
XXI. A Letter from Lewis Weston Dillwyn, Esq. addressed 
to Sir Humphry Davy, Bart. P. R. S. 
Read March 25, 1824. 
My Dear Sir, 
Through you I beg leave to offer to the Royal Society, 
some further observations on the relative periods at which 
different families of testaceous animals appear to have been 
created, and on the gradual approximation which may be 
observed in our British strata, from the fossil remains of the 
oldest formations to the living inhabitants of our land and 
waters. 
The series of strata beginning with transition lime and 
ending with lias, contains shells belonging to various genera 
of conchifera, cephalopoda, annelides and herbivorous tra- 
chelipoda; and also some other shells, as for instance, the 
multilocular and spiriferous bivalves, which cannot be referred 
to either of those natural orders, or groups of genera, into 
which all the other testacea, both recent and fossil, have been 
divided. In the simple bivalves belonging to these strata, 
the marks which best serve to distinguish their families are 
generally obliterated, and but little more can with any cer- 
tainty be observed, than that the two orders into which La- 
mark has divided them, have existed together throughout 
every formation from transition rocks to the present day. 
An examination of the few perfect specimens which I have 
met with, however, leads me to suspect that all the dimyairia 
