The Relationship of tJie Tctracoralla to the HexacoraUa. 195 



"4. At this stage the axial tabellse are formed, and the central 

 column assumes its true character. The extrathecal region is 

 narrow, and many of the septa are still in contact with the 

 epitheca." 



Summary. 



The appearance of a columella in nearly all the late Paleozoic 

 genera of Tetracoralla shows that this line was still a variable 

 one. The cause which effected this widespread change must 

 have been some important alteration in function or structure in 

 the center of the polyp. The different forms of both parietal and 

 essential columellas indicate some such common underlying 

 change. Finally, it is considered to be significant that this central 

 structure, which is so common in Mesozoic and later genera of 

 HexacoraUa, appeared so often in the tetracoral line just before 

 the Permian-Triassic gap in the record which separates the two 

 groups. 



