156 



IV. I. Robinson, 



in the development, it is evident that it really represents two 

 septal elements and so the stage at which the first bilateral pair 

 appears is called the four-septal stage. The first bilateral pair is 

 quickly followed by a second one which appears on the opposite 

 side of the calyx, bringing the number of septa up to six. A 

 pause in the development occurs at this time, and after this the 



Fig. 2. The development of a Carboniferous Zaplirentid. X/^. (After 

 Carruthers.) 



septa are added in a pinnate manner, their inner ends joining 

 the septa of the first and second bilateral pairs. Whether or not 

 this pause indicates an important stage in the phylogeny of the 

 Tetracoralla is not clear, as such phenomena may be explained 

 in various ways, but it occurs at a decided break in the ontogeny 

 of the individual and so must be considered as probably repre- 

 senting a former adult stage. 



Fig. 3. The development of Cyathaxonia (i, 2, 3, 4) and of Turbinolia 

 (i, 2, 3, 5, 6). The first tvvelve septa appear in the same order and with 

 the same arrangement. X/^- (After Faurot.) 



