64 



OUR COMMON BRITISH FOSSILS. 



those formed since then. Again, the septa, or radi- 

 ating plates, of the rugose corals are in multiples of 

 four^ whilst those of aporose corals are in multiples of 

 six. Besides this means of distinguishing the aporose 

 corals from any of the others, the fact that they 

 never have /^^///^— that is, are not divided into hori- 

 zontal layers — is another important distinction. When 



Fig. 45. — Horizontal section of Litho- 

 strotion PhilUpsii (Carboniferous 

 limestone). 



Fig. 46.— Vertical section oi Amplextis 

 coraUozdes, showing the tabnlce. 



the tabulate corals have only faint traces of septa, 

 we can still see they are in multiples of four, and they 

 thus show their structural relationship to the R7cgosa. 

 Dr. Sorby has shown that the tabulate corals are built 

 up of calcite, whilst the Neozoic and modern corals 

 are formed of that limy structure known as arragonite. 

 It may be that the Rugosa are descended from the 



