22 



THE BRITISH SPONGES. 



crease in the newer oolitic group ; and they appear to 

 constitute the most abundant fossils of the cretaceous or 

 chalk formations. * " The upper part of the chalk through- 

 out a large portion of England," says Mr De La Beche, 

 " is characterized by the presence of numerous flints, more 

 or less arranged in parallel lines. "f These hard homogene- 

 ous flints, wliich are usually in the form of figs, pears, or 

 of nodulous masses, are, it may be conjectured, the indu- 

 rated remains of compact siliceous sponges ; as the silice- 

 ous grains and spicula mixed with the chalk itself appear 

 to come from their disintegi'ation or dissolution. 



• De la Beche's Geological Manual, pp. 271, 297, 333, and 456. 

 t Geological Manual, p. 259. 



Fig.^. 



Branchiotoma SrOXGILL.^., Westw. 



