28 
for the substance. of the shells, of which there are no ves- 
tiges remaining. Other genera are often included, and 
to be comprehended also by casts only. 
The existence of these hollow casts, contrary to the 
siliceous forms of the shells found in the green sand, is a 
curious fact that requires to be further attended to : some- 
thing similar occurs among the Crag fossils. 
