160 



BULLETIN : MUSEUM OF COMPAEATIVE ZOOLOGY. 



formed only Avhere the living polyps and other reef builders are always 

 covered by water. This does not necessitate their always being below 

 low tide level, for corals grow constantly in tide-pools, and Serpulae often 

 build up rims enclosing basins whose surfaces are left several feet above 

 low tide. In such basins corals may flourish above sea-level. But while 

 these cases ai'e not at all uncommon, the ai'ea covered by them, when con- 

 sidered in relation to the whole area of the coral reefs of which they form 

 part, is very small indeed. 



'^'?^-~;Cl 



■■■^■ 



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Fig. 



High Kock, Maceio coral reef. From a pliotograph by F. Ambler. 



Along the entire coast from Parahyba do Norte to Parcel das Paredes, 

 the Abrolhos, and Coroa Vermelha, the coral reefs not only rise above 

 low tide water over enormous areas, but the upper sui'faces of all these 

 exposed reefs are dead, excepting in so far as they are made of such 

 things as grow in tide-pools. In the north the coral reefs are universally 

 bored by mollusks, sea-urchins, etc., and for the most part have an 

 etched appearance. 



In the State of Alagoas, three kilometres south of Sao Miguel dos 

 Milagres and north of the Rio Camaragibe, there are exposures, at low 

 tide, of coral reefs upon the sandy shore. At the mouth of a small 

 stream the reef has been covered by sand, and later it has been again 



