FOSSIL CORALS. 69 



British rocks, especially the Carboniferous limestone, 

 are in places almost entirely composed of corals, reef- 

 building, deep-sea, and shore-loving species. 



There is often a difficulty in recognizing which 

 of the fossil corals were "reef-builders," and which 

 were not. For it does not 

 follow that because the fossil 

 corals are of a compound 

 character they were therefore 

 engaged in the work of reef- 

 building. Perhaps the safest 

 plan is to trace the existing 

 genera of reef-builders as far 

 back in geological time as we 

 can, or at any rate to compare the fossil kinds with their 

 nearest living representatives. Few genera are more 

 distinctively " reef-builders " than the Astrcea, whose 

 characteristic star-like arrangement of polypes or 

 corallites (the latter often so close together that they 

 press each other into oval or polygonal shapes), has 

 given to this genus its distinctive name. The wide- 

 spread geographical distribution of the genus Astrcea, 

 and the fact that it is engaged, in areas separated 

 by enormous geographical distances, in reef-building, 

 would be an incidental proof to a geologist of its 

 geological antiquity, even if this genus were not found 

 in our Upper Silurian and Devonian limestones. Thus 

 Astrcea rotulosa (Fig. 50) is a living species of this 

 interesting genus of corals found abundantly in West 



