398 



CORAL-KAG. 



[Ch. XX. 



m an upward direction, there is no similar discordance as we proceed 

 downwards, and pass from one to another of the several members of 

 the Jurassic group, the Upper, Middle, and Lower Oolite, and the 

 Lias. Thus, for example, I find on consulting Mr. Etheridge's tables 

 of British Fossils,* that of sixty species of all classes that lived in the 

 period of the Kimmeridge clay, twenty, or about 33 per cent., pass 

 down into the Coral-Rag ; or, if we confine our attention exclusively 

 to the mollusca, of thirty-three species in the Kimmeridge clay, eight, 

 or 24 per cent., are common to the Coral-Rag. 



MIDDLE OOLITE. 



Coral-Rag. — One of the limestones of the Middle Oolite has been 

 called the "Coral-Rag," because it consists, in part, of continuous 

 beds of petrified corals, for the most part retaining the position in 

 which they grew at the bottom of the sea. In their forms they more 

 frequently resemble the reef-building -poliparia of the Pacific than 

 do the corals of any other member of the Oolite. They belong 

 chiefly to the genera Thecosmilia (fig. 391), Protosseis, and Tham- 

 nasfrcea, and sometimes form masses of coral 15 feet thick. In the 

 annexed figure of a Thamnastrcea (fig. 392), from this formation, it 



Fig. 391. ; 



Fig. 392. 



Thecosmilia annularis, Milne Edw. and J. Hainie. 

 Coral-rag, Steeple Ashtc-n. 



Thamnastrcea. 

 Coral-rag, Steeple Ashton. 



will be seen that the cup-shaped cavities are deepest on the right- 

 hand side, and that they grow more and more shallow, until those 

 on the left side are nearly filled up. The last-mentioned stars are 

 supposed to represent a perfected condition, and the others an imma- 

 ture state. These coralline strata extend through the calcareous hills 

 of the N.W. of Berkshire, and north of Wilts, and again recur in 

 Yorkshire, near Scarborough. The Ostrea gregarea (fig. 393) is very 

 characteristic of the formation in England and on the Continent. 



* Compiled for a work entitled " Stratigraphical Arrangement of British Fos- 

 sils," now preparing for publication by Mr. Etheridge. 



