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these two great divisions of the Silurian period, considered in a 

 palfeontological point of view, are as distinct from each other as 

 the Devonian is from the Carboniferous period. In North 

 America, where the Silurian rocks are fully developed, the same 

 grouping holds true ; and Professor Dana has well expressed the 

 physical by a statement of the palseontological fact, when he 

 says: — In the Lower Silurian era at least 1000 species of 

 animals became extinct in America, and 600 in Great Britain, 

 and in the Upper Silurian, 800 or more in America — clearly 

 proving how distinct the fauna of the Lower is from that of the 

 Upper Silurian rocks. This description of the change in the 

 fauna as a whole is found to apply with equal truth to the class 

 now under consideration. 



In the Lower Silurian rocks. Corals are not abundant: 

 Favosites, NehuUpora, Halysites, Heliolites and Petraia date from 

 the Bala beds. Petraia rugosa was the first fossil I found 

 in the Ash beds of the Bala series on the summit of Snowdon. 



In the Trenton Limestone, Lower Silurian of North America, 

 some ZoANTHARiA TABTJLATA attained a great development, as 

 Choetetes Lycoperdon and Columnaria alveolata; masses of the 

 latter, consisting of congeries of columns one-sixth of an 

 inch in diameter, are found, which weigh between two and 

 three thousand pounds ; this Coral, therefore, was one of the 

 great reef-builders in the Lower Silurian seas. 



In the Upper Silurian rocks. Corals are very abundant, and 

 constituted an important part of the marine faima of the period. 

 The number of species is likewise considerable, and the very 

 fine state of preservation in which most of the specimens 

 are fCTund, renders their study comparatively easy, and the' 

 determination of the species most satisfactory. 



It is in the Wenlock Limestone, however, that Corals 

 are most abundant ; ancient reefs of considerable extent and 

 thickness are seen in this formation at May HUl, Tortworth, 

 Dudley, Wenlock Edge, Walsall, and other locahties. 



With a few exceptions the Reef-building Corals of the Silurian 

 age belong to the Zoantharia tabtjlata, and the Zoantharia 

 RUGOSA. The species found in British strata resemble those 



