Ch. XVII] FOSSILS OF THE UPPER CRETACEOUS BEDS. 



249 



are dispersed indifferently through the soft chalk and hard flint, and some 

 of the flinty nodules owe their irregular forms to inclosed sponges, such as 

 fig. 285 a, where the hollows in the exterior are caused by the branches 

 of a sponge, seen on breaking open the flint (fig. 285 b). 



Fig. 285. 



A branching sponge in a flint, from the white chalk. 

 From the collection of Mr. Bowerbank. 



Fig. 2S6. 



Siphonia pyri- 

 formis. 



Chalk marl. 



The remains of fishes of the Upper Cretaceous formations consist 

 chiefly of teeth of the shark family, of genera in part common to the 



Fig. 283. 



Fig. 287. 



Palatal tooth of 



Piychod-us deeurrens. 



Lower white chalk. 



Maidstone. 



Cestracion Phillippi ; recent. 

 Port Jackson. Buckland, Bridgewater Treatise, pi. 2T, d. 



tertiary, and partly distinct. To the latter belongs the genus Ptychodus 

 (fig. 287), which is allied to the living Port Jackson Shark, Cestracion 



