30 J. E. DUERDEN. 



phyllum represent a developmental stage when compared with 

 the mature condition of such a form as Strcptelasma ; as in the 

 case of the alar fossulae, a developmental stage in Streptdasma 

 persists as the adult condition in Hadrophyllum. The axial car- 

 dinal fossula of Hadrophyllum. is thus shown to be associated 

 with two groups of incompletely developed septa, a group on each 

 side of the cardinal directive septum, while at the same time the 

 latter is much smaller than the other primary septa. Such a 



a. 



a 



FIG. 9. Exosepta (.r) have now appeared in association with most of the fully 

 developed septa, namely, all the protosepta (except the ventral directive) and the 

 metaseptal pairs a and b. The exosepta are at first turned towards and fused with 

 their corresponding dorsal entoseptum. 



fossula as a whole may be spoken of as compound, while those 

 in the two alar regions are simple fossulae. 



Studies on other species of rugose corals show that throughout 

 the group the cardinal fossula in the adult calice is constituted 

 either on the Hadrophyllum or Strcptelasma plan ; rarely, as in 

 Anisophyllum, the cardinal directive septum is much larger than 

 the other primary septa and then the fossula is really double, a 



