166 



FORMATION OF CORAL REEFS. 



verge toward the central opening, as in the As- 

 traeaiis, 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 Astraeans. The Porites re- 

 semble the Astraeans, but the pits are smaller, 



with fewer partitions and fewer tentacles, and 

 their whole substance is more porous. 



