FOSSIL SPONGES, ETC. 21 



sequently the study of the chalk-sponges is intimately 

 connected with the origin of chalk-flints, and perhaps 

 also with that of " chert." 



No one can work at these flints long without 

 seeing that the external shapes of the flint-nodules 

 are very often determined by the shapes of the fossil- 



Fig. 12.— Common Branching Sponge, imbedded in flint-nodule. 



sponges they enclose. In many instances the surfaces 

 of the flints are marked by Indications of the enclosed 

 fossils cropping out. This is particularly the case 

 with the Ventriadites and the Spongites (as Mantel 1 

 called them). The latter are the branching sponges 

 which cause the flints to be "rotten." In the pits 



