PART II. CHAPTER XVII. 



219 



Fossils of Oolite Group. 



Fig. 209. 



Fig. 210. 



Cast of Diceras arietina. 

 Coral rag. 



Cidaris coronata. 

 Coral rag. 



rated marls crop out from beneath the gault; and, on decomposing, 

 leave the surface of every ploughed field literally strewed over 

 with fossil oysters. 



One of the limestones of the Jura, referred to the age of the 

 English coral rag, has been called " Nerinsean limestone" (Cal- 

 caire a Nerinees) by M. Thirria; Nerincea being an extinct 

 genus of univalve shells, much resembling the Cerithium in 

 external form, and peculiar to the oolitic period. The annexed 

 section (Fig. 207.) shows the curious form of the hollow part of 

 each whorl, and also the perforation which passes up the middle 

 of the columella. N. Goodhallii (Fig. 208.) is another English 

 species of the same genus, from a formation which seems to form 

 a passage from the Kimmeridge clay to the coral rag.* 



A division of the oolite in the Alps, regarded by most geolo- 

 gists as coeval with the English coral rag, has been often named 

 " Calcaire a Dicerates," or " Diceras limestone," from its con- 

 taining abundantly a bivalve shell (see Fig. 209.) of a genus 

 allied to the Chama. 



Among the characteristic shells of the Inferior Oolite, I may 

 instance Terebratula spinosa (Fig. 211.), Pholadomya fdi- 



Fig. 211. 



Fig. 212. 



Terebratula spinosa. 

 Inferior Oolite. 



a Pholadomya fidicula. Inferior Oolite. 



b Heart-shaped anterior termination of the same. 



* Fitton, Geol. Trans., Second Series, vol. iv. pi. 23, fig. 12. 



