PART II. CHAPTER XV. 



197 



Different Cliaracter of Chalk in South of Europe. 



Fig. 173. Two individuals deprived of their opercula, adhering together 



174. Same seen from above. 



175. Transverse section of part of the wall of the shell, magnified to show 



the structure. 



176. Vertical section of the same. 



On the side where the shell is thinnest, there is one external furrow and cor- 

 responding internal ridge, a. b. Figs. 173, 174. ; but they are usually less promi- 

 nent than in these figures. This species has been referred to Hippurites, but 

 does not, I believe, fully agree in character with that genus. I have never seen 

 the opercular piece, or valve, as it is called by those conchologists who regard the 

 Rudistes as bivalve mollusca. 



But this family, which is so feebly represented in England 

 and the north of France, becomes quite characteristic of rocks 

 of the cretaceous era in the south of France, Spain, Greece, and 

 other countries bordering the Mediterranean. 



Fig. 177. 



I &1a?'valve of H. radiosa, \ ^ower chalk, South of France. 



Fig. 178. Fig. 179. 



Spharulites agariciformis. Hippurites organisans. Desm. Pyrenees. 



Between the region of chalk last mentioned in which Perigeux 

 is situated, and the Pyrenees, the space B intervenes. (See Map.) 



Here the tertiary strata cover, and for the most part conceal, 

 the cretaceous rocks, except in some spots where they have 

 been laid open to view by the denudation of the newer forma- 

 tions. In these places they are seen still preserving the form of 

 a white chalky rock, which is filled in part with grains of green- 

 sand. Even as far south as Tercis, on the Adour, near Dax, it 

 retains this character where I have examined it, and where M. 

 Grateloup has found in it Ananchytes ovata (Fig. 158.), and 

 other fossils of the English chalk, together with Hippurites. 

 When we arrive at Bayonne and the Pyrenees, the cretaceous 



E * 



