124 M. K. A. Zittel on Fossil LitMstidce. 



tidse are to be obtained almost unaltered. We have merely 

 to treat the fragments of rock with dilute muriatic acid to 

 have before us in a short time the whole skeleton in perfect 

 beauty. In the White Chalk of England and France, also, 

 Lithistidfe, especially of the genus Siphonia ( Ghoanites) , some- 

 times occur which show the skeletal elements in excellent 

 preservation enclosed in a crust of flint; but in these the 

 canal-system is filled with a mealy siliceous substance, which 

 cannot be removed by treatment with acid. 



The above-mentioned skeletons behave, when examined 

 microscopically, exactly like recent Lithistidse. They possess 

 the same optical properties as the latter in Canada balsam, 

 resins, and glycerine. But this favourable state of preserva- 

 tion only occurs rarely. 



In England the White Chalk of Flamboi'ough Head ap- 

 pears to furnish the most numerous Lithistidaj ; but although 

 these specimens, after treatment with muriatic acid, show all 

 the external characters of the sponge-body, and especially the 

 canal-system, in wonderful beauty, they are but little adapted 

 to microscopic examination. The individual skeletal elements, 

 which are usually united to form fibres, are almost always 

 soldered together by an accession of silica, more or less con- 

 verted into crystalline silica, and so much altered that we can 

 only exceptionally succeed in determining their original form. 

 Certain specimens from the Coral Rag of Nattheim, and the 

 Upper Jurassic strata of Muggendorf and Amberg, in the 

 Franconian Jura, also behave in the same way. 



A different process of silicification has taken place in most of 

 the Lithistidae from the Middle and Upper Cretaceous of 

 France (Touraine, Normandy), as also in many from the 

 North-German Cretaceous. In these the skeleton is certainly 

 often well preserved ; but flint has penetrated into all its 

 interstices, so that it is useless to think of isolating its indivi- 

 dual parts. Examination with a good power under the micro- 

 scope leads most quickly to a determination in such cases ; but 

 for a more thorough investigation thin sections must be pre- 

 pared. Under certain circumstances, however, fine translucent 

 chips will suffice. 



In Brunswick (near Boimtsdorf and Gliesmarode) Lithis- 

 tidfB, preserved in the above manner and penetrated with flint, 

 occur in great abundance in a derivative deposit (Diluvium). 

 The skeleton is often of a dark colour and here and there 

 somewhat decomposed, but in the main well preserved and 

 capable of being shown in thin slices. Most of the Cretaceous 

 sponges of Touraine present similar characters. In the latter, 

 however, the process of decomposition has not unfrequently 



