114 ON THE SILICIOUS BODIES OF CHALK, ETC. 



presented an even surface, such as would have been produced had a 

 portion of fluid matter entered though the ambulacra and subsided, as 

 water or any similar liquid would have done ; but, on the contrary, the 

 surface was always undulated, and frequently projected considerably 

 above the surrounding parts, more especially near the side of the shell, 

 against which it was frequently built in semi -cones or columns, and in 

 the space thus unoccupied by the flint there was always included one or 

 both of the large orifices of the shell. The undulated surface of the 

 flint thus concealed within the Echinite, presents exactly the same orga- 

 nic characters which are observed on the flint nodules. Most fre- 

 quently the Echinite is filled with the flint ; and the animal having thus 

 built its prison full, has usually perished from want of sustenance ; at 

 other times it has survived this incarceration, and has grown out of one 

 or both of the great orifices of the shell, and has then, in some cases, 

 increased to a very considerable extent. On the exposed surface of the 

 whole of these, whether it be only to the extent of a slight convex pro- 

 jection from the orifice of the Echinite, or to a considerable mass, 

 an accordance will be found with the organic characters before des- 

 cribed. 



If some of the specimens of Galerites and Spatangus, which are filled 

 with flint, be placed in diluted muriatic acid, and the whole of the shell 

 be removed, the appearance presented by the silicious casts will still fur- 

 ther corroborate the opinion of their spongeous origin. Occasionally it 

 will be found, that the ambulacral orifices of the shell have been filled 

 with fine threads of silex, and that these are based upon the cast ; but 

 more frequently we shall find, that opposite to each of these numerous 

 minute orifices, there is a small but deep depression, the interior of 

 which presents the usual characteristic surface observed on all flints, 

 and the minute tubuli will be seen as boldly projecting at the bottom 

 and round these small excavations, as upon any of the exposed surfaces 

 of the flint." In these cases the ambulacral 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 depres- 

 sions thus produced directly beneath them, were clearly intended as a 

 means of facilitating this operation. Cn the surface of the cast, in the 

 immediate neighbourhood 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. 



