E 3 
deftroyfed, but its place is fupplied by fpar 
is filled with Free Stone. 
This very much refembles the Mj/a piElorum^ 
Lin: (a fhell found in great plenty recent in the 
river Avon, which runs between the hills in the 
environs of Bath) excepting that the fhell here 
figured is thicker in proportion to its breadth. 
I am not acquainted with any names which I could 
with propriety give to any of the Jhellsfrom Fig. 17, 
to Fig. 22. inclvfve. 
Valves 
* This happens when water containing in it vitriol, or 
other-like falls, pervades any fraia ; it diffolves the fhells 
lodged in fuch firata by little and little, carries their dif- 
folved particles away with it, and leaves the /paces, be¬ 
fore filled and pofTefled by thofe fielts, empty; fo where 
it happens, that the water, paffing through, carries with 
it, befides fuch falls, particles fpar, or other mineraJs, 
it frequently lodges them in thofe cavities, and there 
leaves them till at laft it fills them up. Wherever this 
happens, it always follows, as of necefiity it muft, that 
the matter of fpar, or other minerals fo formed, exhibits 
and reprefents the very fizes, and pcrfeQ; figures, interior, 
and exterior, of the fhells whofe places it had filled. 
Woodward’s Nat, Hift, of the Earth illuftrated and 
hilarged, p. 30, 
