160 JULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
formed only where 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 are not at all uncommon, the area 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. 
Fra. 88. High Rock, Maceio coral reef. From a photograph by Е. Ambler. 
Along the entire coast from Parahyba do Norte to Parcel das Paredes, 
the Abrolhos, and Согда Vermelha, the coral reefs not only rise above 
low tide water over enormous areas, but the upper surfaces 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 Alagóas, three kilometres south of São 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 
