FORMATION OF CORAL REEFS. 8? 
Polyzoa, and certain plants called Nullipores, the Madrepo- 
raria in the main are the true reef-builders. They are con- 
fined to waters in which through the coldest winter month 
" 
the temperature of the water 
does not fall below 68° F., 
though usually the waters are 
much warmer than this, the 
mean annual temperature be- 
ing about 734° F. in the North 
Pacific and 70° F. in the 
South. Coral reefs are abun- 
dant in the West Indies, but 
still more so in the Central 
Pacific, where there are a 
much greater number of spe- 
cies of corals (Dana). Along 
the Brazilian coast, as far 
south as Cape Frio, are coral 
reefs (Hartt). In depth living 
coral-reef-builders do not ex- 
tend more than fifteen or 
twenty fathoms below the sur- 
face. 
Coral reefs are divided by 
Dana into outer or barrier 
reefs (Fig. 57) and inner reefs. 
The barrier reefs are formed 
from the growth of corals ex- 
posed to the open seas, while 
the inner or fringing reefs 
(Fig. 57) are formed in quiet 
water between a barrier reef 
and the island. As coral 
reefs are usually built upon 
islands which are slowly sink- 
ing, barrier reefs are simply 
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ancient fringing reefs formed when the island stood higher 
above the sea, hence they are built up as rapidly as the land 
sinks, and thus the top of the reef keeps at the level of 
