262 M. K. A. Zittel 07i Fossil HexactinelUda. 



found that in many cases half or a small portion of a sponge- 

 body could be admirably prepared by treatment with dilute 

 acid, whilst the whole remainder of the mass, although appa- 

 rently of the same constitution, was completely dissolved. By 

 the preparation of thin slices it was ascertained that such com- 

 pletely soluble sponges in fact possess a skeleton consisting 

 of calcspar. 



In the Franconio-Swabian and Swiss Jura, sponges which 

 perfectly agree morphologically with the living Hexactinellida 

 are more frequently furnished with a calcareous than with a 

 siliceous skeleton. At other localities, on the contrary, as 

 near Nattheim, Oerlingen, Muggendorf, Engelhardsberg, &c., 

 the sponges, like all the other fossils, appear in a roughly 

 silicifted state, which, however, has evidently been produced 

 under the influence of the process of fossilization. 



In the Upper Cretaceous deposits of Ahlten, Linden, and 

 Lemforde, in Hanover, and of Coesfeld, in Westphalia, the 

 [lexactinellid sponges are likewise almost without exception 

 siliceous. But while the skeletons from Nattheim and the 

 last-mentioned Jurassic localities have acquired a semicrystal- 

 line, rough texture, and, when examined under the microscope, 

 no longer exhibit the finer structural characters (such as axial 

 canals and ornamentation of the fibres), the Cretaceous forms 

 behave under the microscope exactly like macerated skeletons 

 of living Hexactinellida. 



A corresponding constitution is also possessed by the sili- 

 ceous parts of those sponges in which a portion of the skeleton 

 consists of calcspar, or which are derived from beds in which 

 calcareous and siliceous skeletons occur together. Here belong 

 especially the forms from the true SjDongitenkalk of the White 

 Jura, y and 2, in Swabia and Franconia. In such cases it can 

 hardly be determined beforehand whether Avhen treated with 

 acids the skeleton will be completely destroyed or beautifully 

 macerated. The siliceous skeletons here obtained, sometimes 

 in a beautiful state, sometimes quite fragmentary, yield but 

 little to the living Hexactinellida as regards the preservation 

 of the finest ornamentation and the distinctness of the axial 

 canals. Optically, however, they show a peculiar behaviour. 



Thus, if small fragments obtained by corrosion or thin 

 slices be mounted in Canada balsam, or in any other resin 

 with a similar coefficient of refraction, a very indistinct image 

 is obtained when they are examined by the microscope. The 

 outlines are not sharply marked, and scarcely any of the more 

 delicate ornamentation makes its appearance. The object 

 stands out but little from the enclosing substance, and evi- 

 dently presents exactly the same conditions of refraction as 



