MORPHOLOGY OF THE MADREPORARIA. 



37 



depression on each side of the large axial septum corresponding 

 with the smaller, newly added septa. 



As shown in the various figures the cardinal or ventral-direc- 

 tive septum is smaller than the other principal septa, and remains 

 thus when all the other septa have become equal ; the axial fos- 

 sula is then represented in the fully developed calice by the smaller 

 cardinal septum alone, without a special lateral depression on 



JC. 



x. 



FIG. 10. Section towards the upper part of the corallite. The septa are now free 

 from the columella, and the ventral directive septum is smaller than the others. More 

 exosepta and metasepta have appeared than in Fig. 9, each exoseptum being still 

 fused with its corresponding entoseptum. 



each side. Such a condition clearly calls for a different expla- 

 nation from that given for the alar fossulae, or for the lateral 

 parts of the cardinal fossula ; these have been found to corre- 

 spond with ontogenetic stages of species which attain nearly per- 

 fect radiality in the adult, while the ventral-directive septum in 

 most instances remains shorter even within the mature calice. 

 Manifestly the small directive septum must correspond with some 



