H. Iv. A. Zittel on Fossil Hexactindlida. 265 



tion, we find that the trabeculaj, which cross each other at 

 right angles and form cubical meshes, consist of crystallized 

 calcspar. In their general form the calcareous parts of the 

 skeleton are undistinguishable from the siliceous parts ; but 

 while in the latter the axial canals are most distinctly pre- 

 served in tlie interior of the trabeculte, the calcareous parts 

 prove to be completely .solid : the calcspar forms a homo- 

 geneous, undifferentiated mass. The absence of the axial 

 canals in the calcareous and their presence in the siliceous 

 parts of one and the same sponge-body, appear to me to fur- 

 nish incontrovertible proof that silica formed the original 

 substance of the skeleton, and that the Hexactinellida and 

 Lithistida composed of calcspar have changed their chemical 

 constitution only in consequence of the process of fossilization; 

 calcspar has consequently taken the place of the amorphous 

 silica which was originally present. 



This somcAvhat unusual pseudomorphosis implies that pre- 

 vious to the penetration of the carbonate of lime the siliceous 

 skeleton was dissolved and carried away. Considering the 

 comparatively easy solubility of amorphous silica in water 

 impregnated with alkaline substances, this process presents 

 nothing startling, especially if we consider what an extended 

 surface the siliceous parts, furnished with axial canals and 

 composed of concentric layers, presented to the solvent. 



The occurrence of Hexactinellida and Lithistida in which 

 cither a part or the whole of the siliceous framework has been 

 removed, and in which the place of the siliceous fibres is occu- 

 pied by hollow tubules, forming a network of meshes in the 

 penetrated mass of rock, is, in fact, quite an ordinary pheno- 

 menon. Toulmin Smith has already described examples of it 

 from the White Chalk of England ; and this state of preserva- 

 tion shows itself still more frequently in the sponges from the 

 ferruginous and sandy Upper Chalk of Saratow, in Russia. 

 By treatment with dilute acid, separate and, indeed, generally 

 small parts of the skeleton of such sponges are beautifully 

 macerated, whilst by far the greater part of the fossil is 

 entirely dissolved. Examination Avitli the lens then shows 

 immediately that in the soluble parts the siliceous framework 

 is replaced by fine cavities, which furnish a true picture of 

 the original skeleton*. 



Not unfrequently the cavities produced by the removal of 

 the siliceous fibres are entirely or partially filled with rust- 



* Upon this point see Rosen, ' Ueber die Natur der Stroniatoporen,' 

 Dorpat, 1807, pp. 10 et neq. That the explanation of thi.s state of pre- 

 senation attempted by Rosen is erroneous, must appear from the above 



statements. 



