396 THE MEDALS OF CREATION. Cuap. XI. 
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 au 
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. 
A small oyster, called Ostrea vesicularis, is a characteristic 
shell of the chalk; one valve is convex, the other flat; it is 
abundant in the Chalk of Norfolk, and also in the Fire- 
stone of some localities: it is figured Ly. p..212. Ano- 
ther small species, having the margin plicated (O. phicata), 
is also frequent in the Chalk. A large shell, with the 
margins deeply indented by angular folds, resembling the 
recent cockscomb oyster, is abundant in the Chalk Marl and 
Firestone; particularly near Dover, and around Selbourne 
in Hampshire, where it attracted the notice of Waits, by 
its resemblance to the: living “ Cockscomb Oyster” of the 
West Indies ; it is named Ostrea carinata, and figured Ly. 
p. 212, fig. 204. One other species may be noticed, the 
Ostrea deltoidea, which has been found in every locality of 
the Kimmeridge Clay in England and France. It is a very 
flat species, and of a triangular form; the specific name is 
derived from a supposed resemblance to the Greek letter A, 
delta. 1 believe that in England no shells of this genus 
have been observed in strata older than the Lias. 
GrypHHa. Lign. 127, fig. 6.—The shells to which the 
term Gryphea, or Gryphites, is applied, are related to the 
Oyster, but distinguished by the deep concave under valve, 
and its curved summit, or beak, and the almost flat, or 
