TABLE OF CONTENTS 



PAGE 

 lEKiMINOLOGY , jrj 



Summary 1^3 



Importance of the Question of Primary Septa 



Theories of Previous Writers 154 



Conclusions 157 



The Two Theories of the Origin of Hexacoralla 



Theory of Common Ancestry 159 



Theory of Direct Descent 160 



Tests of the Theory of Direct Descent from Tetra- 

 coralla 

 Geological Occurrence 



So-called Paleozoic Hexacoralla 

 Palceacis and its Relation to the Tabulata . .161 

 The Genus Calostylis and its Position in the Classi- 

 fication 173 



Other Peculiar Forms Sometimes Called Hexa- 

 coralla 178 



Summary of ''Paleozoic Hexacoralla" . . . 181 

 First Fossil Corals 



Mackenzia of the Middle Cambrian . . . 181 



First Coral Fauna 182 



Probable Ancestry of the Tetracoralla . . . 182 

 Dominance and Decline of Various Coral Stocks . 185 

 Evolutionary Trends in Tetracoralla . . . 186 



Plasticity of Tetracoralla in the Late Paleozoic 

 The Columella as an Example of Variability 



Morphological Significance . ... • 188 



Steps in Development 190 



Summary I95 



Bibliography . . 196 



