PETRIFIED OYSTER.
Lign. 20:—Oyster from the Chalk, near Brighton (natural size).
Occasionally the soft body of the mollusk completely silicified—that is, transmuted into flint—is found in its natural position in the shell. A beautiful example of this kind is represented in [Lign. 20]. It is an extinct species of oyster: both valves were entire when I removed the chalk and cleared the specimen; part of one valve has been broken away to expose the petrified body of the animal. I have seen a Trigonia[AB] from the oolite of Tisbury in Wiltshire, in which the entire body of the mollusk was transformed into flint, and the branchiæ or lamellated gills were beautifully defined, though converted into semi-transparent chalcedony.
[AB] Trigonia: a genus of bivalves, of which there are many extinct species in the chalk and oolite; some bands of Portland stone are an aggregation of Trigoniæ: a few very small species, inhabitants of the seas of Australia and New Zealand, are the only known living forms of this once prevailing type of mollusca. See 'Medals of Creation,' p. 407.
[Note II.] [Page 17.] Wood in Flint.
WOOD IN FLINT.