MORPHOLOGY OF THE MADREFORARIA. 



95 



ventral succession, but in two series - - first a series of six (lll.a 

 Ill.r) within the dorsal of the two interspaces in each sextant,, 

 and then the remaining six (III.C/-III/) in a like order but within 

 the ventral of the two interspaces. As explained below, the 

 exosepta (X.) constituting the last outermost cycle have not the 

 same ordinal significance as the entosepta. 



lla 



Hid 



Ilia 



FIG. 9. Diagram showing the order of appearance of the septa in a corallite with 

 three cycles of entosepta ( I. -III. ) and an outer cycle of exosepta ( X. ). The Roman 

 numerals indicate the cycle to which the septa belong and the letters their sequence 

 in the cycle. 



Studies on the mesenterial sequence of other corals indicate 

 that a similar septal succession will in all probability be followed 

 by most forms in which the adult calice shows a regular hex- 

 ameral cyclic plan. Individual departures from the order may 

 be expected, but are to be looked upon as irregularities ; regu- 

 larity of growth of the higher cycles of mesenteries and septa is 



