PORIFERA. 37 



sponges are also common in some places ; (3) in the 

 Lower Chalk of the South of England, the Calcisponges 

 are however not represented here ; (4) in the Upper 

 Chalk, where the siliceous sponges are very common, the 

 Calcisponges rare. Lastly in the Tertiary formations few 

 sponges have been found. 



