20 



never replaced by silica. Hence portions of the tests of Ino- 

 ceramua are often seen quite unaltered in the middle of 

 a flint. 



Here is a beautiful example of a sponge partly converted 

 into chalcedony. The tubules of the original sponge, were the 

 moulds into which the flint was formed, and the long twigs as 

 it were of chalcedony now represeut them. Nature has kindly 

 allowed us, one might say, to see some of the steps of this interest- 

 ing process of the conversion of a sponge into a flint, as if to 

 whet our appetite for more information concerning it, but has 

 carefully hidden from us that which we most desire to know. 



In many of these chalcedonic nodules where the proce?s has 

 gone further, some of the spicules and sponge fibre of the once 

 living organism, may still be discerned. Why are these left ? 

 By what process was all the rest dissolved ? For it is only 

 occasionally that we meet with spicules or fibre in these flints 

 which must have derived their silica from vast quanties of the 

 spicule and skeletons of sponges. 



Ventriculites. — " These are cup-shaped or vasi-form 

 sponges, hollow within, freely open above, and below tapering tc^ 

 a point which is encloFed in a surrounding sheath of fibres •" 

 (Sollas). These sponges belong to the same great order as tht- 

 recent Euplectella and Hyalonema, specimens of which are 

 exhibited, and in the latter may be seen the sheath of fibres 

 similar to that by which the ventriculite was anchored to the 

 bottom of the sea. Only one species of a modern sponge has 

 been discovered as yet that in its structure is allied to the 

 ventriculites, and that is represented by a single insignificant, 

 specimen {Mylhisia Grayi) in the British Museum. It differs 

 however, widely in form from them, and it may be said that the 

 ventriculites which were so abundant in the seas of the 

 Cretaceous epoch have left no direct representatives. The curious, 

 forms which they now assume as flinty concretions have obtained 

 for them the name of fossil mushrooms, fossil figs, &c. 



Choanites. — Perhaps the most beautiful forms which a flint 

 pebble when cut and polished presents to us are associated with 

 Choanites — sponges of the genus " Siphonia ■" as they are now 

 termed — the fossil sea-anemones of some shop windows. They 

 are particularly characteristic of our Brighton beach, many haviug 

 been, no doubt, derived from the old beach now to be seeL 

 embedded in the chalk cliffs between Black Eock and 

 Rottingdean. The siliceous matter has often aggregated round 

 them in that beautiful translucent form which is termed chalce- 





