RECENT AND FOSSIL. 101 



or osciilum, and is evidently very yonnp: ; had it obtained its full 

 development it would probably have entirely inclosed the fragment 

 of Inoceramus, as is the case in our next specimen. Here we find 

 that the sponge has grown to greater dimensions, and wherever it 

 lias covered the shell we have flint, and there only. In like manner 

 the empty shells of sea-urchins or echinites have afforded favour- 

 able resting-places for sponge-gemmules ; in some instances the 

 sponge has partly filled the shell, and in others wholly filled it and 

 even crept round the exterior. Eut, again, wherever the sponge 

 has extended, we have flint, and there only. 



In further proof of these echinoderms having been inhabited by 

 sponges which subsequently became fossilized, we may note the 

 rows of depressions or pits found in the flints, which I consider to 

 be due to the fact of the inhalation of water by the living sponge 

 through the ambulacral foramina. We can well understand that 

 when a sponge has completely or to a great extent filled the shell 

 of an echinite, it should make use of these openings for the purpose 

 of inhalation, and that a corresponding depression on the surface of 

 the sponge should be the result. On the contrary, we often find, 

 instead of depressions, small columns of flint filling up these am- 

 bulacral pores. In such cases I infer that the sponge has had a 

 sufiicient supply of water without making use of the ambulacral 

 foramina, and this supply may fi'equently be traced to a crack in 

 the shell of the echinus. 



Layers of thin flint exist in many chalk-pits, which, having no 

 external appearance of sponges, have been cited by the opponents 

 of the spongeous origin of flints as proving that sponges were not the 

 nuclei of flints ; but from the discoveries of Wallich, Carpenter, 

 and others, it appears that a layer of protoplasm or sarcode exists 

 at the bottom of the present ocean, and thus a clue to the forma- 

 tion of these flint-layers becomes apparent, for doubtless the de- 

 composition of this layer of sarcode precipitated the silex of the 

 ocean in the same way that the decomposition of the sarcode of 

 the sponges would do, and thus produced those extensive layers 

 of flat flints to which I have referred. 



Mr. Frederic Kitton, of Norwich, writing on the " Spongeous 

 Origin of Flints," * says that silica under certain conditions is 

 soluble in water to a considerable extent, and that in the earlier 

 epochs of the world, silica might have been present to a larger 

 extent than at this time. " The presence of silica in a state of 

 solution being," he says, "an ascertained fact, there is nothing 

 improbable in the hypothesis that sponges should have formed 



the nuclei of these flinty concretions Another and still 



more effectual cause of the elimination of silica would be the 

 decomposition of the sarcode and keratode material ; as this goes 

 on certain gases are produced, and the silex precipitated from the 

 solution." 



A paper on the process of silicification of animals, read before 



* ' Trans. Norfolk and Norwich NaturaUsts' Soc.,' vol. i, p. 57. 



