SUPPLEMENTARY INDEX TO VOL. IT. FAD 
loans, a far greater number of individual shells, diversified 
as much as possible as to their localities, and not forgetting 
in any instances to name and precisely describe these localities; 
without which additions, fossil shells are of no real value 
for improving the present infant state of geological 
knowledge. 
In the extended comparison of shells, named by you or 
Mr. Smith, with their places and strata, to which I have 
already alluded, [ have been concerned to find, according 
to the best opinion I can form, from the local facts men- 
tioned by you and Mr. Smith regarding them, and what 
I know of the ranges of the several strata, and of the 
distribution of fossil shells in their peculiar beds, from 
the experience I have gained in such quarries, banks, 
pits, canals, wells, &c. which produce them, almost 
throughout Great Britain, that no less than 104 of the 
shells (including some varieties) already named or 
described in the three works mentioned, should, for 
useful geological purposes, be made to form 279 species, 
each with its own compound name ; or at least, that these 
279 shells of as many distinct beds* of the strata (excepting 
here any errors in the recorded facts) should be distin- 
guished, by the usual addition of Greek letters, as distinct 
varieties. 'This latter plan I have adopted in the Strati- 
graphical Index to Vol. If. instead of merely adding ?’s, 
as I did in the former Index ; and for the information of 
your Readers hereon, I beg the favour of you to insert 
as follows, the names of all the described shells, Xe. 
which as far as I can yet judge, require these marks, to 
distinguish the varieties of different strata, viz. 
* It may be proper to keep in view, that I mean by this term, (as 
all practical men do) the thinnest natural divisions of Strata; and not 
in any case thick masses, or whole mountains, although of one mineral 
species, as some writers imply by its use. 
