M. K. A. Zittcl on Fossil Hexactvnellida. 263 



the Canada balsam. But, on the contrary, if we treat the 

 same object with glycerine or water, the image leaves nothing 

 to be desired in the matter of clearness and sharpness. All 

 ]3reparations derived from such sponges must therefore be 

 mounted in glycexine. Many Cretaceous sponges from the 

 North-German and Bohemian Planer, in which' also only par- 

 ticular parts of the skeleton still consist of silica and remain 

 after the action of muriatic acid, behave in the same way. 



With living siliceous sponges, as also with fossil skeletons 

 from the above-mentioned localities of the Cretaceous formation, 

 where the hexactinellid sponges always present a siliceous com- 

 position, Canada-balsam is the best medium for mounting pre- 

 parations. On the other hand, if living or Cretaceous sponges 

 which furnish beautifully clear images in Canada balsam are 

 put into glycerine, the same phenomenon is observed as when 

 the Jurassic sponges are treated with Canada balsam. The 

 image loses all its sharpness and clearness ; and, indeed, under 

 some circumstances it is so obliterated that with tolerably 

 bright illumination scarcely any thing can be seen. 



in correspondence with these remarkable phenomena, there 

 is also a divergent behaviour under polarized light. All 

 recent siliceous sponges, whatever the form of the parts of the 

 skeleton may be, consist of simply refractive, amorphous silica. 

 This is the case also with many Cretaceous forms, which must 

 be mounted in Canada balsam. But if we bring a preparation 

 of the Jurassic forms above described, mounted in glycerine 

 or water, under the polarizing apparatus, the most vivid colour- 

 phenomena are obtained on turning the Nicol's prism. The 

 fragments of the framework or spicules show the spotted many- 

 coloured iridescence so characteristic of quartz, and, indeed, 

 often quite as strongly as small quartz-grains, which may 

 have got accidentally into the preparation. I know of no 

 other similar optical behaviour in organized silica. Some- 

 times, indeed, in Diatom ace» or Badiolaria, we observe 

 faint colours on the rotation of the prism, but never a 

 phenomenon of such intensity as in the case of these Jurassic 

 or Cretaceous sponges. Between this decidedly doubly 

 refractive silica and the unaltered amorphous silica there are, 

 however, many stages of transition. The siliceous sponges 

 from the Quadratus chalk of Linden, near Hanover, for exam- 

 ple, are visible both in Canada balsam and in glycerine ; and 

 with these, in a certain position of the prism, slight colour- 

 phenomena are also obtained in the polarizing microscope. 



This singular optical behaviour indicates distinctly that in 

 the older siliceous sponges a physical alteration has occurred, 

 by which the originally singly refractive silica has been con- 



19* 



