io OUR COMMON BRITISH FOSSILS. 



has decomposed, there is left behind the skeleton, 

 formed by the interlacing of the above-mentioned 

 spicules. 



We may practically regard these " spicules " as 

 having been crystallized within the soft, white-of-egg- 

 like sponge-flesh. They are not always of the same 

 chemical composition indeed they rarely are, even 

 in the same species or individual. But there is 

 generally an exclusive majority of spicules of the 

 same chemical or mineral composition. The fibres 

 are formed of a substance chemically resembling that 

 which enters into the composition of horn, hair, wool, 

 or feathers, and which is characteristically an animal 

 production, called chitine. Our washing-sponge is 

 the skeleton of the Chitinous group. Their skeletons, 

 being usually fibrous, could hardly be expected to be 

 found in the fossil state, unless they underwent a 

 change. Two other classes of sponges are recognized 

 by naturalists, and by some are named according to 

 the substance of which the spicules of the skeleton 

 are composed. Thus, in one group the spicules 

 are formed of lime, and so the term calcareous is 

 applied to them. Ten species of this sort of fossil 

 sponge are found in the Ludlow rocks. In the other 

 kind, the spicules are formed of silica or flint, and 

 accordingly this group of sponges is termed siliceous. 

 The last group possesses the most complex and 

 beautifully shaped spicules, and the dead skeleton 

 formed by them is one of the most lovely of 



