FORMATION OF CORAL REEFS. 165 
especially adapted to give a strong, solid base to 
the whole structure, are known in our classifica- 
tions as the Astreans, so named on account of 
the star-shaped form of the little pits crowded 
upon their surface, each one of which marks the 
place of a single more or less isolated individual 
in such a community. 
Thus firmly and strongly is the foundation of 
the reef laid by the Astreans; but we have seen 
that for their prosperous growth they require a 
certain depth and pressure of water, and when 
they have brought the wall so high that they have 
not more than six fathoms of water above them, 
this kind of Coral ceases to grow. They have, 
however, prepared a fitting surface for different 
kinds of Corals that could not live in the depths 
from which the Astreans have come, but find 
their genial home nearer the surface; such a 
home being made ready for. them by their prede- 
cessors, they now establish themselves on the top 
of the Coral wall and continue its growth for a 
certain time. These are the Meandrinas, or the 
so-called Brain-Corals, and the Porites. The Me- 
andrinas differ from the Astreans by their less 
compact and definite pits. In the Astreans the 
place occupied by the animal in the community 
is marked by a little star-shaped spot, in the cen- 
tre of which all the partition-walls meet. But in 
the Meandrinas, although all the partitions con- 
