The Relationship of the Tetracoralla to the Hexacorolla. 155 



forms, calling attention to the geological occurrence, and to 

 certain peculiarities of coral evolution — tendencies which are now- 

 referred to as parallel development. 



Von Koch (1896) argued that the Tetracoralla arose from a 

 stock whose individuals had six primary elements. 



Van Beneden (1897) believed the fourfold structure of Tetra- 

 coralla indicated their close relationship to the Scyphomedusse. 



Up to 1902 the idea of a primitive six-rayed condition in 

 Tetracoralla had not received much support from paleontologists, 

 but in 1902 Duerden, using new methods of careful sectioning, 

 found a stage in LopJiophyUum proliferum with only six septa. 

 He concluded that this was the earliest skeletal stage. (See 

 Fig. I.) 



Fig. I. The development of Lopliophyllutn prolife'rum. This and the 

 following diagrams represent transverse sections taken in a continuous 

 series from near the tip of the corallum to the calyx. The cardinal septum 

 in each case is above; the counter septum below. X Yz- (After Duerden.) 



Gordon (1906), working with decalcified siliceous specimens, 

 announced the discovery of a stage with four septa in Streptel- 

 asma profundum. This precipitated an argument which devel- 

 oped new interest in the problems of morpholog}^ and ontogeny 

 in both living and fossil forms of Anthozoa. 



Carruthers (1906) finally settled the matter by finding in 

 Zaphrentis phillipsi stages with "one," four and six septa (see 

 Fig. 2). It was found by a comparative study of a number of 

 sections that the stage with "one" septum was quickly succeeded 

 by a stage in which two new septa appeared as a bilateral pair. 

 As the single "primary septum" is broken by a central gap later 



