thefe being more ponderous the n common Oyfter-ffiells, 
and fomewhat near the fpecifick Gravity of the Sekmtes. 
But indeed they differ one from another in Gravity, as 
well as from other Shells, as they partake more or lefs 
of a Tophaceous SubfVance that coats many of em 
on the infide, and vi^hich perhaps may be fomewhat 
akin to the Sekmtes, And whether they may not have 
many other very different qualifications one from ano« 
ther, according to the feveral different Beds they are 
found in, I think there may be (bme reafon to doubt. 
I have obferved fome fuch differences among the Corma - 
Ammonis , having had one or two fmall ones from our 
Coal- pits here, that had a confiderable mixture of the 
Pyrites ; whereas thefe that are found about Whttly^ ap- 
proach , I think , more to the nature of the Alum- 
(tone I and perhaps the Cornua Ammonis of the Ancients 
were found in Beds of fomewhat yet more valuable ; 
fince Tliny fays they were o§ a Golden Colour, and were 
reckoned,/«/^r Sacratiffimas /Ethiopia gemmas. I know. 
Agricola, [De ortu& caufis Sulterraneorum, lib.iv.] ac- 
counts for this Golden Colour after another manner: 
Cornua Ammonis inquit fucco aluminis tnfeEla aurei color is 
fimt. And I am ready enough to think,that there is fome 
truth not only in this Obfervation,but in what he imme- 
diately d^MsJdem inquit & aliis quilufdam lapidtlus acci* 
dit. For I cannot but attribute the extraordinary appea- 
rance of Colours in the Teacock tail Coaly to its being 
infeded with the Succus Aluminis ^ having feenfome pie- 
ces of this pretty fort of Coal, Ihoot into true and ge- 
nuine Alijm. Yet I cannot take this to be all the rea- 
fon of the Golden Colour of the Cornua Ammonis of the 
Antients, fince I think, if this had been all, they had 
never been nurabred, . or deferved a place among their 
Gems. 
But 
