26 CORAL REEFS. 
Northern Coral (Astrangia).—This beautiful coral 
may be found in Long Island Sound, near New Haven, 
and on the New Jersey shore. The polyps are pure white, 
Fic. 25.—Astrea palliéa (living). 
standing high above the cells. The tentacles are covered 
with lasso-cells, each about 7$y of an inch in length. It 
thrives well in the aquarium. 
Coral Reefs.—These are banks or shoals of dead or 
living coral at or below the surface. The tops of sub- 
merged hills and elevations gradually approach the surface 
by the accumulation of organic matter, principally from 
the continuous falling* of shells of Rhizopoda and Fora- 
minifera (Fig. 3), and other forms, until finally a platform 
of limestone is built that reaches within forty or fifty yards 
of the surface. Now, the reef-making corals, Madrepores, 
etc., that do not flourish in deeper water, become fixed, 
grow, and accumulate, with Gorgonias and other forms, 
until they reach the surface. Seeds, perhaps of the man- 
grove, now obtain a footing, and the reef in time becomes 
a coral key or island. 
* It has been estimated by Murray that, if lime-secreting organisms 
are as numerous down to a depth of six hundred feet as they are near 
the surface, there would be more than sixteen tons of calcareous shells 
or carbonate of lime in the uppermost one hundred fathoms of every 
square mile of the ocean, 
