Geological Society. 



243 



Por several reasons the Author chooses the corals and brachiopods 

 as zone and sub-zone fossils, and he has selected genera for zone- 

 indices and circuli (or species-grouj^s) for sub-zonal indices. A 

 circulus is defined as an aggregate of all the species which possess, 

 in common, a large number of essential properties, and are the 

 results of similar chains of evolution. To secure definiteness 

 photographic figures are introduced, not only to illustrate new 

 specific names, but to convey the precise sense in which well-known 

 specific names are employed in the paper. The relative acceleration 

 of the two groups employed is not identical in different localities, 

 and there is a small relative displacement of one group upon the 

 other, even within the area considered by the paper. The strata in 

 which the indices of two successive zones are found to overlap one 

 another are referred to as ' horizon o, /3, y,' etc. 



The detailed stratigraphical portion of the paper deals with all 



* Employed throughout the preliminary working system. 



