16 OUR COMMON BRITISH FOSSILS. 



The discovery that many sponges which originally pos- 

 sessed flinty or siliceous spicules have had them con- 

 verted into lime a most unexpected metamorphosis, 

 but one which cannot be gainsaid has greatly per- 

 plexed naturalists as to which were formerly calcareous 

 and which siliceous sponges. Thus Zittel, the great 

 authority on fossil sponges, regards those which are 

 so abundant in the Coral Rag beds at Farringdon, 

 as calcareous ; whereas Carter thinks they were ori- 

 ginally siliceous sponges whose spicules have been 

 chemically changed. 



This only shows how involved are the problems 

 dealing with the most simple and rudimentary of 

 marine animals. But the fact remains, that sponge- 

 life has not materially altered from the very earliest 

 period of our planet's history until now. There is a 

 much greater variety of species in recent seas than 

 perhaps was ever the case before, although it is 

 doubtful whether sponges are as abundant in any 

 of our great seas or oceans as when the European 

 Chalk was deposited. 



Fossil sponge-hunting becomes most interesting 

 when we come to the Cretaceous or chalk formation. 

 True, we find fossil sponges in all marine calcareous 

 and arenaceous formations, from the Silurian upwards. 

 Even fresh-water sponges, closely allied to that now 

 abundant in rivers and streams {Spongilla fluvia- 

 tilis), are known to have been in existence during the 

 Purbeck limestone stage of the Secondary epoch. 



