8 . INTRODUCTION. 



Siliceous Sj)icular Structure dissolved, leaving enijiti/ Moulds. — This condition is 

 of frequent occurrence in beds where the matrix is suitable for retaining the form of 

 the spicular mesh, and it furnishes a clear proof of the ready solubility of siliceous 

 sponge-structure. The siliceous skeleton of these sponges has been entirely removed, 

 so that there are only empty cavities in the place of the solid spicular arms and 

 nodes. These moulds are sometimes present in a matrix of chert or flint, of which 

 there are examples in the Silurian strata of North America and in the Upper Green 

 Sands of Wiltshire ; they also occur in compact Jurassic limestones, in the Cambridge 

 Green Sand, and even in the Upper Chalk of the south of England. It is somewhat 

 surprising that the empty moulds of the delicate sponge-skeleton should have been 

 retained in the soft Chalk of the south of England ; but they have been so perfectly 

 preserved that in many instances the moulds of the delicate balks or rods which form 

 the lantern-like nodes of Hexactinellid sponges remain intact. Not only aie the 

 connected skeletons of Hexactinellid sponges thus dissolved, but the detached spicules 

 of Tetractinellid sponges, though relatively much larger and more robust, have also, in 

 many instances, suffered a similar fate. A noted example occurs in beds of the Hils 

 sandstone in the north-west of Germany, where the moulds of the spicules remain in 

 a matrix of semi-translucent chert, giving to it a porous aspect. Other instances are 

 present in Lower Green Sand chert from near Haslemere in Surrey. In some of the 

 specimens from this latter locality the spicular canal had become infilled with silica 

 before the solution of the spicule itself, and this solidified canal now remains as a 

 delicate rod in the central axis of the hollow mould. In some sponges from the 

 Upper Green Sand the spicular skeleton seems to have been incrusted with a thin 

 delicate film of silica, which now remains after the solution and removal of the 

 skeleton. In some examples of Miocene Hexactinellid sponges described and figured 

 by Manzoni * the skeleton is similarly incrusted with a siliceous pellicle, but it has 

 not been subsequently dissolved. 



Changes produced in Calcareous Sjionges ly Fossilization. — As a rule the mineral 

 structure of fossil calcareous sponges has undergone fewer changes than the structure 

 of siliceous sponges, though, as a result of the small dimensions of the calcareous 

 spicules and their very intimate disposition in the fibre, a very slight amount of 

 alteration has been sufficient to obliterate the form of the individual spicules, and to 

 merge them into a common fibrous mass of amorphous or crystalline calcite. In 

 general the calcareous sponges imbedded in arenaceous strata are better preserved 

 than those from calcareous beds. In the numerous calcareous sponges from the 

 Neocomian gravels of Earringdon, in Berkshire, the sponge-fibre is enveloped in a 

 coating of dog-tooth crystals of calcspar. The microscopic structure of these sponges 

 is very imperfectly preserved. The fibre of the calcareous sponges from the Upper 

 Green Sand of Warminster is of a soft earthy character with a greyish-white tint, and 



* Spugne silicee del Miocene medio, t. 5. f. 15. 



