FOSSIL SPONGES, ETC. 23 



canals within represent the entombment of sponges 

 which have decayed, all but their spicules, now form- 

 ing the " flint-meal." The most " rotten," or sponge- 

 bearing chalk-flints I ever worked upon are those of 



Fig. 17. Ventriculite. 



Flamborough Head, in Yorkshire, and Trimingham, 

 in Norfolk. In both cases the fossil sponges are of 

 the kind called Lithistid, the hexactinellid sponges 

 being rare. The external shapes of lithistid sponges 

 vary to an enormous extent, even in the same genus. 



