THE GRAPTOLITES AND CORALS 



301 



septa can easily be recognized in slightly later stages,, and 

 by means of a series of transverse sections the method 

 by which additional septa are intercalated can be recog- 

 nized : this method shows that the symmetry of the coral 

 is bilateral, not radial (Fig. 90). One of the six primary 

 septa is the cardinal septum, already noted; another 

 which is and remains exactly opposite to this is called 

 the counter-septum. The two on either side of this are 

 the counter -lateral septa ; the two between them and the 

 cardinal are the alar septa. 



FIG. 91. SEPTAL DEVELOPMENT IN RUGOSA. 



Diagrammatic side views of apex, a, Showing cardinal septum in centre, 

 metasepta arising on both sides of it ; b, showing alar septum in 

 centre, cardinal on right margin, counter on left, metasepta arising on 

 only one side of alar septum, away from cardinal. 



Professor Duerden distinguishes the six first septa as 

 protosepta, and the major septa that arise later as 

 metasepta. As growth proceeds, additional major septa 

 appear between the cardinal and alar septa, and between 

 the alar and counter-lateral, but none between the 

 counter-lateral and counter-septum (Fig. 91). There are 

 thus four spaces only in which new septa appear, and the 

 newest septum in each space is always that nearest the 

 cardinal septum. The minor septa, on the other hand, 

 appear simultaneously after the full number of major 

 septa has been formed. 



