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those found in chalk seem to be somewhat injured at the end a, where they are deficient, and are rounded, but have an uneven surface, as if they had been gnawed or eaten by the pholas. Those found in clay near Piddington [1] Oxfordshire, approach nearer to the fusiform kind, and have a different appearance at the smaller end f; where the laminæ are reduced to a white impalpable powder, by corrosive juices in the earth, so as to stain the fingers when first taken out; and they afterwards retain a white chalky appearance: but, amongst a great number, I never found one that was three inches long. These have suffered in the same manner as Fig. iv. Fig. 3. f, shews where the alveolus terminates: g, b. g, h. how much has been destroyed by vitriolic acids [2]. At Stons-field they are found much longer than at Piddington, and are inclosed in stone, which is split by the workmen to make slates. Here we often find
- ↑ Fig. 3.
- ↑ It may be asked, why one part suffers more than another, as all parts are homogeneous, and free from extraneous mixtures? My answer is; because those parts, where the concamerated shell is lodged, are much thinner than the other parts of the Belemnite; and consequently the walls are more easily broken down, and the alveolus, being still less solid, is sooner destroyed, and reduced to an impalpable powder, by vitrolic and other acids, which the water takes up as it passes through different strata, abounding more or less with pyritical matter. Where no spar follows the acid, the parts are carried away and lost in the interstices of the earth, and a mould or plasm is left, which Steno calls an aërial shell. See his Prodomus, pag. 84. But where the spar abounds, it pervades the whole substance, fills up the cavity, and assumes the true form of the shell; and sometimes, by bursting the pores, is so far substituted in the place of the original particles, that the several diaphragms, with the siphunculus of the alveolus, are acurately and nicely preserved.
