94 THE FAUNA OF THE DEEP SEA 



tions laave been successive, each joeriod of tlieir history 

 sending some specimens to survive or to become 

 extinct in the struggle for life in the deep sea. 



Of the Madreporarian corals, several genera are 

 now known to inhabit very deep water, but they do 

 not present many very remarkable points of diver- 

 gence from the shallow- water forms. 



It is true that as we pass from the shallow^ waters, 

 of those parts of the world where the great colonial 

 madrepores build up the greater part of the vast coral 

 reefs, into the deeper water beyond them, the solitary 

 forms become relatively more abundant, but no new 

 groups characterised by any special deep-sea attributes 

 make their aj^pearance. We must remember, not only 

 that a great many solitary corals occur in shallow 

 water in different parts of the world, but that some 

 colonial forms, such as Lojjhohelia irroUfera for ex- 

 ample, are found only in very deep water. 



Until quite recently it was usually stated in works 

 dealing with the structures of coral reefs that the 

 so-called reef-building corals, that is to say the large 

 madrepores, astrseids, and others, are confined to water 

 not deeper than thirty fathoms. This limit must 

 now be somewhat extended, in consequence of the 

 discovery by Captain Moore of an abundance of 



