166 FORMATION: OF CORAL REEFS. 
verge toward the central opening, as in the As- 
treeans, these central openings elongate, run into 
each other, and form waving furrows all over 
the surface, instead of the small round ‘pits so 
characteristic of the Astreeans. The Porites re- 
semble the Astraans, but the pits are smaller, 
with fewer partitions and fewer tentacles, and 
their whole substance is more porous. 
