150 Preface 



ing the summer of 191 5 the writer was employed in making thin 

 sections of Paleozoic corals in the Yale collection. Genera were 

 chosen which had resemblances to Hexacoralla, with the hope of 

 finding some which showed a septal arrangement like that of 

 modern corals. As the work progressed, it became increasingly 

 noticeable that there was a great uniformity in the general 

 method of addition of the septa ; that the application of Faurot's 

 rule of quadriseptate arrangement could be quite generally made ; 

 that, in short, the Tetracoralla are a natural group differing in 

 a definite structural phenomenon from all Mesozoic and later 

 forms in which the ontogeny of the skeleton is known. 



Attention was then turned to those Paleozoic forms which 

 have been classed recently with the Hexacoralla. ^^'hile the 

 writer does not pretend to have settled the question of the origin 

 of the Hexacoralla, it is hoped that the evidence here presented 

 will be conclusive in showing that there are no known Paleozoic 

 Hexacoralla, and that the data furnished by this study of Pale- 

 ozoic forms favor the theory of direct descent of modern hexa- 

 corals from tetracorals. 



