M. K. A. Zittel on Fossil Lithistidce. 125 



gone further : in thin sections we observe only isolated well- 

 preserved skeletal elements, between which lie an immense 

 number of blackish or reddish-brown spherules (probably of 

 hydrated peroxide of iron), which are sometimes quite irregu- 

 larly dispersed, but sometimes have undoubtedly got into and 

 completely fill the empty forms of previously existing skeletal 

 elements which have been washed out. 



In the English White Chalk and also in the neighbourhood 

 of Rouen, amorphous flint-nodules occur in great quantities, 

 from which, when split, beautifully preserved sponges are 

 frequently set free. The sponge- body is enveloped by a white 

 porous crust of decomposed flint. Between this and the sponge 

 there is usually a thin layer of snow-white siliceous dust, in 

 Avhich there are numerous well-preserved sponge-spicules. The 

 sponge-body itself either exhibits the state of preservation 

 already described in the case of the Lithistidse of Flamborough 

 Head, or, still more frequently, its interior is completely filled 

 with a homogeneous mass of flint, in which all sponge- 

 structure is destroyed ; in thin slices it appears as a homo- 

 geneous amorphous substance. The surface of the sponge, 

 however, as well as all the parts covered with white siliceous 

 powder, are generally excellently preserved, and are particu- 

 larly well adapted for examination by direct light. 



A less favourable state of preservation of the silicified 

 Lithistidge is that in which the original skeletal elements have 

 been dissolved and carried off", and are now replaced by cavi- 

 ties in the siliceous mass, furnishing a more or less true 

 negative picture of the skeleton which formerly existed there. 

 Numerous specimens from Touraine, from the White Chalk 

 of England, from the Greensand of Ilegensburg, and the Coral 

 Rag of Nattheira, Gingen, Muggeudorf, and Amberg show 

 this phenomenon. 



Similar " negative " skeletons, not, however, enveloped in 

 flint but in phosphatic glauconitic calcareous sand, occur in 

 the Upper Cretaceous of Saratow in Russia, where the cavities 

 are also sometimes filled with brown ironstone. I have already 

 called attention to this state of preservation (which also occurs 

 in the Hexactinellidse), in the first section of these " Studies." 



Lithistidse in which the original siliceous skeleton is re- 

 placed by rust-coloured hydrated peroxide of iron occur 

 very frequently in the Mucronatus- and Quadratus-c\\sl\. of 

 Schwiechelt, Peine, and Vordorf in Brunswick, sometimes near 

 Alilten in Hanover, in the White Chalk of France, also in the 

 North-German, Bohemian, and Saxon Planer, and frequently 

 in the Franco-Swabian Jura. 



Lastly, we have still to mention the calcified lithistid skele- 



