[ 47 ] 
thofe found in chalk feem to be fomewhat injured 
at the end where they are deficient, and are 
rounded, but have an uneven furface, as if they had 
been gnawed or eaten by the pholas. Thofe found in 
clay near Piddington [/], Oxfordfhire, approach nearer 
to the fufiform kind, and have a different appearance 
at the fmaller end f where the laminae are reduced 
to a white impalpable powder, by corrofive juices 
in the earth, fb as to ftain the fingers when firft. taken 
out ; and they afterwards retain a white chalky ap- 
pearance : but, amongft a great number, I never 
found one that was three inches long. Thefe have 
fuffered in the fame manner as Fig. iv. Fig. 3. 
fhews where the alveolus terminates : g, h. g, b. 
how much has been defrayed by vitriolic acids f u ]. 
At Stons-field they are found much longer than at 
Piddington, and are inclofed in ftone, which is fplit 
by the workmen to make flates. Here we often find 
CO FI S* 3* 
[zz] It may be afked, why one part fufters more than another,., 
as all parts are homogeneous, and free from extraneous mixtures? 
My anfwer is ; becaufe thofe parts, where the concamerated {hell 
is lodged, are much thinner than the other parts of the Belem- 
nitej and confequentiy the walls are more eafily broken down, 
and the alveolus, being ftill lefs folid, is fooner deftroyed, and re- 
duced to an impalpable powder, by vitrolic and other acids, which 
the water takes up as it paffes through different ftrata, abounding 
more or lefs with pyritical matter. Where no fpar follows - 
the acid, the parts are carried away and loft in the interftices 
of the earth, and a mould or plafm is left, which Steno calls an 
aerial {hell. See his Prodomus, pag. 84. But where the fpar 
abounds, it pervades the whole fubftance, fills up the cavity, and 
affumes the true form of the fhel! ; and fometimes, by burfting 
the pores, is fo far fubllituted in the place of the original particles, 
that the feveral diaphragms, with the fiphunculus of the 
alveolus, are acurately and nicely preferved. 
them * 
