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



verted into doubly refractive silica. Whether this phenomenon 

 has been produced by strong fission and numerous small clefts 

 or by a change in the position of the molecules, as in the slow 

 conversion of amorphous into crystalline sugar, or whether 

 chemical influences have intervened in this case, I am unable 

 at present to decide *. At any rate, however, the optical pro- 

 perties of the above-mentioned fossil siliceous sponges prove 

 that a certain alteration has taken place in the substance of 

 the skeleton. This fact is further confirmed by their constitu- 

 tion in other respects. 



When examined by direct light, they do not appear with a 

 bright glassy lustre and transparetnt, like living siliceous 

 sponges, but dull, white, and opaque. Of the thin concentric 

 layers of which the siliceous structures of the sponges are 

 composed, nothing can be detected even under the highest 

 powers ; and the whole surface is more or less corroded by 

 innumerable small depressions and elevations. It is true that 

 by means of Canada balsam or glycerine the originally turbid 

 fragments may be rendered completely or more or less trans- 

 parent, but still without attaining the limpidity of the recent 

 glass-sponges. 



As these phenomena are most strikingly observed at those 

 localities where at the same time fossil Hexactinellida orLithis- 

 tida occur with calcified skeletons, the idea of a physical altera- 

 tion presents itself irresistibly. The behaviour with caustic 

 alkalies also leads to the same conclusion. Thus, whilst amor- 

 phous silica dissolves pretty readily in solution of potash or 

 soda, the Jurassic Hexactinellida with double refraction are 

 but little attacked even by strong boiling, and only dissolve 

 after long digestion, when they leave a very small residue. 

 The less altered Cretaceous siliceous skeletons with single 

 refraction are a little more easily soluble. 



The opinion has been orally expressed by those whose 

 opinion is worthy of consideration, that among the fossil 

 sponges there may have been forms which indeed perfectly 

 agreed morphologically with certain existing Hexactinellida 

 or Lithistida, but in which the skeleton was originally com- 

 posed not of silica but of carbonate of lime. 



A microscopic examination of the calcified Hexactinellida 

 at once does away with this notion. In the case of a sponge 

 from the White Jura of Streitberg, for example, half calcified, 

 half siliceous, if we examine a thin slice of the calcified por- 



* Similar optical phenomeua in flint, which, however, certainly, like 

 the singly refractive menilite, has originated from amorphous silica, lead 

 to the assumption of a metamorphosis of the same kind even in the mas- 

 feive siliceous segregations in the chalk. 



