186 Mr. BowERBANK on the Siliceous Bodies of 



in which the flint veins have heen built, it is a natural consequence that this should 

 not be the case. 



Having thus satisfied myself that the common tuberous flints, the horizontal 

 tabular flints, and those forming perpendicular or oblique veins, were all produced 

 by the same agency, and having observed the frequent occurrence of the partial 

 imbedment of shells and other extraneous bodies, I was naturally led to infer, that 

 in all probability the interior casts of Echinites and similar bodies, which are fre- 

 quently found to be filled with flint, were also produced by the same agency ; I 

 therefore procured numerous specimens of silicified Echinodermata, and their ex- 

 amination strongly corroborated this supposition. Some of them were not entirely 

 filled with flint ; in one case a small portion only was siliceous, in others two- 

 thirds or three-fourths of their interior were so occupied, while the remaining 

 space was filled with chalk. Upon clearing away the chalk from this part of the 

 specimen the flint never presented an even surface, such as would have been pro- 

 duced had a portion of fluid matter entered through the ambulacra and subsided, 

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

 face was always undulated, and frequently projected considerably above the sur- 

 rounding 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 organic characters which are observed on the flint nodules. Most 

 frequently 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 shght convex projection from the orifice of the Echinite, or to a considerable 

 mass, an accordance will be found with the organic characters before described. 



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 ap- 

 pearance presented by the siliceous casts will still further corroborate the opinion 

 of their spongeous origin. Occasionally it will be found, that the ambulacral ori- 

 fices 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 

 tubuU will be seen as boldly projecting at the bottom and around these small exca- 



