the Chalk, Greensands and Oolites. * 187 



vations, as upon any of the exposed surfaces of the flint. In these cases the am- 

 bulacral orifices of the shell have evidently been used by the sponge as so many 

 inlets to admit the streams of water which were necessary to its existence ; and the 

 depressions thus produced directly beneath them were clearly intended as a means 

 of facilitating this operation. On the surface of the cast, in the immediate neigh- 

 bourhood of the two large orifices of the shell, there is frequently a series of 

 channels, which have evidently been left by the sponge for the same purpose as 

 the depressions opposite the ambulacral pores, and the bottoms and sides of these 

 channels exhibit the sponge tubuli in a like manner. 



It frequently occurs in the Echinites which are filled with flint, that portions of 

 the shell have been replaced or infiltrated with silex. In all these cases that I have 

 seen, the silex presents a stalactitical or chalcedonic form, and never exhibits the 

 spongeous texture. Very frequently, however, thin laminae of spongeous texture 

 are found to have been built between the plates of the Echinite, where they have 

 happened not to have been quite in contact; and in these laminae the tubuli are as 

 beautifully distinct as in the most favourable portions of the mass of the cast. If 

 the surface of the cast be microscopically examined, we shall frequently observe 

 that the flint has not been in such a state of contact with the shell as a cast from 

 a fluid material would be supposed to present ; for although the boundary of each 

 plate is well marked, the areas of their impressions exhibit such a view of the tu- 

 buli of the sponge as we might naturally expect to find where numerous minute 

 tubes have been built against a flat or slightly concave surface. Sometimes the 

 sponge has grown round the interior of the shell, and has left a hollow near its 

 centre ; and occasionally the sponge appears closely approximating, yet not quite 

 adhering, to the inner surface of the Echinite. In these cases, a thin film of 

 chalcedony is frequently spread over the organized surface of the sponge, which in 

 a specimen in my possession is in several places to be seen through small breaks in 

 the film. 



The results arising from the various forms of flint which have been described, 

 induced me to beheve that the flint cases enveloping the numerous and beautiful 

 specimens of sponges and corals of the Wiltshire chalk would probably prove to 

 have originated in a similar manner to the forms of flint before described. I there- 

 fore requested Mr. John Baker of Warminster, who collects large quantities of 

 these beautiful fossils, to furnish me with some, the interior surfaces of which had 

 not undergone the usual process of washing and brushing. Having cleaned these 

 carefully, by pouring into them a small stream of water, I observed vSpicula pro- 

 jecting from all parts of the interior surface ; and the same appearance was ex- 

 hibited to a greater or less extent in more than twenty specimens which I examined, 



2 b2 



