372 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1962 



Figure 6. — Acetabularia, a handsome, tropical green alga, is shaped like a gay umbrella. 

 Figure 7. — Rhipocephalus is a small, "treelike" green alga found on the Caribbean shores. 



the waves on all sides, provides a relative calm over the inner part of 

 the reef. If the sea level is rising, or the sea floor subsiding, a lagoon 

 develops within the slightly elevated ridge, aided by the destructive 

 process of solution resulting from the action of carbonic acid derived 

 from the respiratory carbon dioxide produced by the life of the lagoon. 



The rate of outward growth of the reef margin reflects differences 

 in surf and in available growth nutrients, due to wind and currents. 

 As a result, the yomig, dynamic atoll usually takes on a somewhat 

 elliptical shape. 



Reef margins are extended seaward in three ways : by the growth 

 and cementing action of the nullipores, by the often profuse growth 

 of corals on the outer front, or face, of the reef below the surf zone, 

 and by the infinitely slow build-up of a vast talus slope of nuUipore 



