MORPHOLOGY OF THE MADREPORARIA. 



39 



of the mesenteries and septa. It is the manner of appearance of 

 the septa beyond the primary six which separates the Rugosa 

 from modern hexameral corals, a separation of the same signifi- 

 cance as that by which zoologists distinguish cyclic hexameral 

 actinians from zoanthid polyps. 



The presence of the smaller axial septum in the cardinal 



O- 



FIG. 12. Arrangement of the septa in a young corallite of Streplelasina profundttni 

 (Owen). At this early stage the septa are arranged in a manner very similar to 

 those in the mature corallite of Hadi-opkyUitm ( cf. Fig. I), while in the fully de- 

 veloped corallite they are nearly radial. The septa show a slight inequality in 

 number on each side, a condition also usually met with in recent zoanthid polyps. 



fossula of the Rugosa serves to confirm in a peculiarly direct 

 manner the relationship of the group to the living Zoantheoe. 

 As is well known, one of the characteristics of the zoanthid polyp 

 is the occurrence of a single gonidial groove, sulcus, or siphono- 



