20 Mr. "SV. J. Sollas on Stauronema, a new 



the iron-pyrites concretions of the chalk seen in miniature ; 

 in size these globules are about ^Ir^ incli in diameter, and 

 may perhaps have formed about the rosettes which surely 

 once existed in the sponge. 



Change in Refractive Index of the Silica of the Fibre. — When 

 fragments of the siliceous network are freed from calcite by 

 means of acid, washed, dried, and mounted in Canada balsam, 

 the fibre is found to be characterized by a remarkable trans- 

 parency, often so great as to render it almost invisible ; and 

 this is perhaps partly to be explained by attributing to it great 

 porosity, by which the balsam would be able to penetrate it 

 everywhere, and great transparency would result ; and this 

 view is supported by the fact that the fibre in the dry state, 

 and mounted in air, appears of a pure snowy white by reflected 

 light. ]iut I scarcely think this is the whole explanation, since 

 when such ])repared fibre is mounted in glycerine jelly, its 

 ti'ansparency is much diminished, and consequently it can 

 be seen with greater distinctness. Now glycerine jelly has a 

 much lower refractive index than Canada balsam ; and hence 

 these different appearances can be readily explained by sup- 

 posing that the silica of the fibre has a refractive index nearly 

 equal to that of the balsam, but higher than that of the glyce- 

 rine jelly. This change in transparency I have found also 

 well exhibited in some beautifully preserved spicules from the 

 Upper Chalk which I hope soon to describe ; these can scarcely 

 be discerned when viewed in balsam, but are seen very clearly 

 in the less-refractive medium. The different appearance of 

 spicules in these different media suggested to me that a corre- 

 sponding advantage might be gained by mounting recent spi- 

 cules in glycerine jelly ; but on following out this idea I found 

 my recent spicules were quite, or at all events nearly, invisible 

 in this material, from which one draws the conclusion that the 

 recent spicules have a refractive index corresponding closely 

 with the lower one of glycerine jelly instead of Avith the higher - 

 one of Canada balsam, and hence, first, that recent spicules 

 are not themselves seen in Canada balsam, but only their 

 negative images or optical casts, and, next, that in process of 

 time the refractive index of spicular silica undergoes an ele- 

 vation approximately equal to that of passing from the refrac- 

 tive index of glycerine jelly to that of Canada balsam. 



Change from the Colloid to the Crystalline State. — The alte- 

 ration in the refractive index would naturally accompany a 

 change of the original silica of the fibre from a colloid to a 

 crystalline condition ; and that such a change has certainly 

 taken place can readily be proved by examining the network 

 as previously prepared, or in an ordinary transparent section, 



I 



