The Relationship of tJic Tctracoralla to the Hcxacoralla. 185 



coralla. A\'e can readily conceive such a primitive structure 

 to have been present, but if in all the other forms of this sub- 

 class the youngest skeletons are found to be equipped with several 

 complete septa, the contention may reasonably be made that 

 such a condition should be found in the ancestral forms. The 

 question would thus be whether in postulating an ancestral genus 

 the more weight should be given to one of two very different 

 forms representing the first known members of the line of 

 tetracorals or to an ontogenetic formula which is found in all 

 except one of the tetracorals so far investigated. In the opinion 

 of the writer the finding of Lindstroomia zvhiteavesi side by side 

 with Streptelasma profundum lessens the significance of geo- 



o 



Fig. 5. Tde twclve-septal stage in StrepteLisma profiinduin (a), after 

 Brown; in 5. coniiciiluin , (!>') diagram, {c) actual specimen. X -3. 



logical occurrence in this case. It seems more reasonable to 

 assume that the siliceous pseudomorphs have failed to preserve 

 the entire structure of vS. projundum accurately, and to trust the 

 unvarying ontogenetic stages to reveal the history of the phylum. 



Dominance and Decline of Various Coral Stocks. 



The three sub-classes, Alcyonaria (Octocoralla), Tabulata, 

 and Tetracoralla, which appeared in the Ordovician, became 

 important members of the later Paleozoic faunas. The Alcy- 

 onaria continued with increasing importance throughout the 

 Ordovician and Silurian and reached a maximum of develop- 

 ment in the Silurian, although they survived the period of stress 

 at the close of the Paleozoic and are represented in the living 

 coral faunas. The Tabulata became very important in the faunas 

 of the Silurian and Devonian due to the remarkable develop- 

 ment of the Favositidse and Halysitidae. In the Mississippian 

 they became an inconspicuous group and disappeared before the 

 end of the Paleozoic. The Tetracoralla were the most con- 

 spicuous of Paleozoic corals, beginning in late Ordovician and 



Trans. Coxn. Ac.vd., Vol. XXI . 13 ^917 



