34 



J. E. DUERDEN. 



whether or not the adult arrangement be radiate ; or, stated in 

 another way, the presence of alar fossulae in an adult corallite is 

 a retention of a developmental characteristic. 



THE CARDINAL OR VENTRAL FOSSULA. 



Enquiry may now be directed into the nature of the cardinal 

 or ventral-directive fossula. This is the most important of the 

 series, being usually represented in mature calices even when all 

 traces of the alar fossulae have disappeared. Further, its con- 

 stitution is by no means so simple as that of the alar fossulae. 



FIG. 7. A third additional metaseptal pair (<) has appeared within both the mid- 

 dle and ventral chambers, the middle pair developing first. A pair of septa (x) has 

 also appeared, one within each of the dorso-lateral primary chambers, fused with the 

 dorsal directive. In relation to the polyp the septa which appear midway between 

 the principal septa are exosepta, and always constitute the outer, smaller cycle in the 

 mature calice ; the larger septa, constituting the inner larger cycle, include both the 

 protosepta and metasepta, and are entosepta. 



As represented in Hadrophyllum (Fig. i) the cardinal fossula is 

 associated with two groups of successively shorter septa (be), a 

 group being situated on each side of the cardinal or ventral 

 directive septum, and, in addition, the cardinal septum (/) is 

 itself smaller than the other fully developed principal septa. The 

 members of each ventro-lateral group of smaller septa (bc) are 

 related to one another in the same manner as are the members 



