ADULT COLONY. 51 



belong to the third cycle and some to the fourth. They have no ordinal value 

 comparable with the entosepta. 



In descriptive works on corals, when giving the number of septa 

 characteristic of any calice, it is usual to consider all the inner cycles as 

 hexamerously complete and then to regard all the missing septa as wanting 

 from the last cycle. Thus Milne-Edwards, in describing the septa of the 

 present species, says : " Three cycles of septa complete, and, in general, a 

 variable number of a fourth cycle." Also Verrill (1901, p. 153): "They 

 [the septa] form three complete cycles, with part of the fourth cycle devel- 

 oped, so that the number is usually 36 to 40." From the relationships here 

 set forth, and more fully discussed in connection with the development of 

 the septa, it is clear that such cyclic plans do not express the true ordinal 

 or morphological relationships of the septa ; the last and penultimate cycles 

 vary in the same degree, and the latter contains both entosepta and 

 exosepta. 



The septal invaginations on plate 6, fig. 34, indicate how the exosepta 

 in most instances fuse with the third septa, but in the two ventral systems, 

 where no members of the third cycle are developed, they unite instead with 

 the secondary septa. In the serial sections the invaginations also show that 

 the fusion of the exosepta with the entosepta is not continuous throughout 

 their vertical length, but is effected only at somewhat regular intervals. 



The section of a fragment of a corallum represented on plate 10, fig. 64, 

 includes the whole of one corallite and portions of the six surrounding 

 corallites, and shows their relationships to one another. The thecal wall 

 separating one corallite from those adjacent to it is seen to be very limited 

 in thickness. At this level in the complete calice twelve septa, representing 

 the first and second cycles, are united directly with the columella. The six 

 primary septa are distinguished by the fact that they extend all the way 

 from the periphery to the columella without fusion with any of the smaller 

 septa. In three of the primary systems there are only three septa a second- 

 cycle entoseptum and two complete exosepta fused with it ; in the remaining 

 three sextants are five septa a second-cycle entoseptum with one exoseptum 

 and a third-cycle entoseptum fused with it, the latter having two exosepta 

 in union with it. 



Comparison may be here made with a somewhat similar horizontal sec- 

 tion of a corallite of S. radians, introduced on plate xv (fig. 12) of Agassiz's 

 " Florida Reefs." Nine distinct, complete septa are represented, and in the 

 interspace between each two occurs a group of three septa, the two lateral 

 united with the middle, which, in its turn, extends to the columella. The 



