Ch. XX.] 



CORALS OF THE OOLITE. 



399 



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

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

 a Nerinees) by M. Thirria ; JVerincea being an extinct genus of uni- 

 valve shells, much resembling the Ceritkium in external form. The 

 annexed section (fig. 394) shows the curious form of the hollow part 

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



894. 



Fig. 395. 



Fig. 



Ostrea gregarea. 

 Coral-rag, Steeple Ashtou. 



Xe rincea hieroglyph lea. 

 Coral-rase. 



Nerinma Goodhallii, Fitton. 

 Coral-rag, Weymouth. I nat. size. 



of the columella. A r . Goodhallii (fig. 395) is another English spe- 

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

 passage from the Kimmeridge clay to the coral-rag.' 4 '' 



A division of the oolite in the Alps, regarded by most geologists 

 as coeval with the English coral-rag, has been often named " Cal- 

 caire a Dicerates," or " Diceras limestone," from its containing abun- 

 dantly a bivalve shell (see fig. 396) of a genus allied to the Chama. 



Fie. 



Fig. 



Cast ef Diceras arietina. 

 Coral-rag, France. 



Cidaris coronata. 

 Coral-rag. 



Oxford Clay. — The coralline limestone, or " coral-rag," above de- 

 scribed, and the accompanying sandy beds, called " calcareous grits," 

 of the Middle Oolite, rest on a thick bed of clay, called the " Oxford 



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



