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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 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. 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

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