In the Astraeans, the multiplication of tentacles is more definite and limited, rising sometimes to ninety and more, though often limited to forty-eight in number, and the transverse floors between the vertical partitions are more complete than in the Fungidae.  The Porites have twelve tentacles only, never more and never less; and in them the whole solid frame presents a complicated system of connected beams.  The Madrepores have also twelve tentacles, but they have a more definite character than those of the Porites, on account of their regular alternation in six smaller and six larger ones; in these also the transverse floors are perfect, but exceedingly delicate.  Another remarkable feature among the Madrepores consists in the prominence of one of the Polyps on the summit of the branches, showing a kind of subordination of the whole community to these larger individuals, and thus sustaining the view expressed above, that the combination of many individuals into a connected community is among Polyps a character of superiority when contrasted with the isolation of the Actiniae;.  In the Sea-Fans, the Halcyonoids, as they are called in our classification, the number of tentacles is always eight, four of which are already present at the time of their birth, arranged in pairs, while the other four are added later.  Their tentacles are lobed all around the margin, and are much more complicated in structure than those of the preceding Polyps.

According to the relative complication of their structure, these animals are classified in the following order:—­

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 connected beams.

Astraeans:  tentacles not definitely limited in number, though usually not exceeding one hundred, and generally much below this number; transverse floors.  Maeandrines, generally referred to Astraeans, are higher than the true Astraeans, on account of their compound Polyps.

Fungidae:  indefinite multiplication of tentacles; imperfect transverse 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 surface of the earth.

  GEOLOGICAL SUCCESSION.

  Present, Halcyonoids. 
  Pliocene, \
  Miocene, } Madrepores. 
  Eocene, /
  Cretaceous, \ Porites
  Jurassic, } and
  Triassic, } Astraeans. 
  Permian, /
  Carboniferous, \
  Devonian, } Fungidae
  Silurian, /