196 AGE OF CORAL KEEFS. 



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. 



ASTRAEANS : tentacles not definitely limited in number, though usu- 

 ally not exceeding one hundred, and generally much below this 

 number; transverse floors. Mseandrinas, generally referred to 

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

 compound Polyps. 



FUKGID^;: 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 of the earth. 



GEOLOGICAL, SUCCESSION. 



Present, Halcyonoids. 



Pliocene, "| 



Miocene, f Madrepores. 



Eocene, 



Cretaceous, ^ 



Jurassic, I Porites 



Triassic, and 



T, . Astraeans. 



Permian, J 



Carboniferous, "j 

 Devonian, r Fungidae. 

 Silurian, 



