£96 AGE OF CORAL REEFS. 
STRUCTURAL {5ERIES. 
HALCYONOIDS: eight tentacles in pairs, lobed around the margin, 
always combined in large communities, some of which are fres 
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. 
AsTrZANS: 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 
Astreeans, are higher than the true Astreeans, on account of their 
compound Polyps. 
FunGiIp#: indefinite multiplication of tentacles; imperfect trans- 
verse beams. 
Actin1#: 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, _ ¢ Madrepores, 
Eocene, 
Cretaceous, : 
: Porites 
Jurassic, 
ee and 
Triassic, 
. Astreans. 
Permian, 
Carboniferous, 
Devonian, } Fungida. 
Silurian, 
