266 M. K. A. Zittel on Fossil Hexactinellida. 



coloured hjdrated peroxide of iron. This state of preserva- 

 tion is especially frequent in the North-German and Bohemian 

 Planer ; rarer in sponges from the White Chalk, the Sand- 

 stone of Saratow, and the White Jura. 



The filling of the cavities with crystallized calcspar occurs 

 chiefly in the Spongitenkalk of the Upper Jura in Switzer- 

 land, Wiirttemberg, Bavaria, and Poland. Here there are 

 localities in which the whole of the siliceous frameworks are 

 entirely converted into calcspar (Wiirgau, Boll, Oberbuch- 

 siten) ; whilst in others [e.g. Schauergraben near Streitberg, 

 Wodna near Cracow) the original siliceous substance is pre- 

 served, but has acquired the optical nature already mentioned. 



If we regard the above explanation of the different states of 

 preservation as correct (and, from the morphological identity 

 of the fossil and living Hexactinellida, other hypotheses which 

 ascribe to the sponges under consideration an originally horny 

 or calcareous skeleton are plainly inadmissible), the question 

 arises, what has become of the dissolved silica of the sponge 

 framework '? 



In the White Chalk this question offers no particular diffi- 

 culties. The occurrence of massive flint nodules is pretty 

 generally explained by a concentration of tlie silica obtained 

 by the lixiviation of siliceous organisms and especially of 

 sponges. In the Spongitenkalk of the White Jura also such 

 siliceous segregations are not entirely wanting, although they 

 are present in less abundance than in the White Chalk. In 

 certain regions, indeed (the sponge-beds of Boll, Streitberg, 

 &c.) , we seek in vain for flint-nodules ; and nevertheless in 

 such localities, side by side with but slightly altered siliceous 

 skeletons, there are numerous specimens converted into calc- 

 ■spar. In such cases the dissolved silica was frequently em- 

 ployed in the silicification of other fossils ; for it is exactly in 

 ■the immediate vicinity of sponges that we find the mest shells 

 of Mollusca and Echinoderms converted into silica. Moreover 

 the dissolved silica appears to be distributed through the rock 

 in another form. Thus in the treatment of entirely or par- 

 tially calcified Jurassic sponges we frequently obtain, in the 

 residue of corrosion, innumerable rounded rough siliceous 

 disks, furnished with deep impressions, or perfectly irregular 

 vermiform bodies *. 



Besides the states of preservation already described, we 

 sometimes find the whole sponge-body penetrated by iron- 

 .pyrites, and partly converted into brown ironstone. Such 



* Such a body is figured by Oscar Schmidt (Spong. Atl. Oc. Taf. ii. 

 fig-. 19). 



