Ostrea, _Lign. 120._--The Oyster is well known to possess no power of locomotion; it is attached to rocks, pebbles, and other bodies, and forms extensive beds, consisting of numerous individuals, of all sizes. There are many fossil species; the British strata yield between forty and fifty. In some localities. Oysters are found in thick beds, of great extent, apparently on the spots they occupied when living. One of the most interesting localities I am acquainted with, is Sundridge Park, near Bromley, in Kent, where a hard conglomerate, entirely made up of oyster-shells, and the shingle that formed their native bed, is quarried. This stone is much employed for ornamental rock-work, and several walls in and near Bromley are constructed of it: these display the fossils, some with the valves closed, others open, others detached, and the whole grouped as if artificially imbedded to expose the characters of the shells. These oyster-beds belong to the tertiary strata of the London basin; they extend to Plumstead, and other places in the vicinity; and in some localities, the oysters are associated with other bivalves, called _Pectunculi_. In the tertiary clays near Woolwich and Bexley, fossil oyster-shells abound. In the neighbourhood of Reading, in Berkshire, an extensive layer of fossil oysters occupies the same geological position, namely, the lowermost sands and clays of the London basin. Wherever the strata around London are perforated to a sufficient depth, this oyster-bed is reached. Very recently an Artesian well was bored at Hanwell, in Middlesex, and at the depth of two hundred and eighty feet this stratum of sand with oyster-shells was found. At Headley, near Reigate, in Surrey, there is a similar deposit. These oysters very closely resemble the edible species. The White Chalk contains several species of Ostrea, but I believe no beds of these shells have been found in it; on the contrary, the shells are diffused promiscuously through the strata. I have collected a few groups of from thirty to forty shells, evidently the young or fry of the species (_O. semiplana_) figured _Lign. 120_. This specimen is an interesting example of the petrifactive process which the mollusca have occasionally undergone; the soft parts of the oyster are transmuted into flint, and the shell is changed into carbonate of lime, having a crystalline structure. Both valves were perfect when discovered, but I chiselled off the greater part of one shell to expose the silicified body of the animal.