196 AGE OF CORAL REEFS. 



STRUCTURAL SERIES. 

 HALCYONOIDS: eight tentacles in pairs, lobed around the margin, 



always combined in large communities, some of which are free 



and movable like single animals. 

 MADREPORES: twelve tentacles, alternating in six larger and six 



smaller ones; frequently a larger top animal standing prominent 



in the whole community, or on the summit of its branches. 

 PORITES : twelve tentacles, not alternating in size ; system of con- 



nected beams. 

 ASTR^EANS: tentacles not definitely limited in number, though usu- 



ally not exceeding one hundred, and generally much below this 



number; transverse floors. Masandrinas, generally referred to 



Astraeans, are higher than the true Astrseans, on account of their 



compound Polyps. 

 FUNGID.E: indefinite multiplication of tentacles; imperfect trans- 



verse beams. 

 ACTINIAE: indefinite multiplication of tentacles; soft bodies and no 



transverse beams. 



If now we compare this structural gradation 

 among Polyps with their geological succession, 

 we shall find that they correspond exactly. The 

 following table gives the geological order in 

 which they have been introduced upon the sur- 

 face ol tho earth. 



GEOLOGICAL SUCCESSION. 



Present, Halcyonoids. 



Pliocene, "J 



Miocene, r Madrepores. 



Eocene, 



Cretaceous, r 



Jurassic, j Porites 



Triassic, 



Permian, 

 Carboniferous, "J 

 Devonian, r Fungidse, 

 Silurian, 



