98 J. E. DUERDEN. 



exosepta indicate that the above formulae do not express the true 

 morphological character of the septa. Any hexameral incom- 

 pletion in the number of septa making up a corallite affects both 

 the entosepta and the exosepta, that is, both the penultimate and 

 the last cycles ; if any septa be wanting to complete the hexam- 

 eral multiple of the last cycle of entosepta the same number 

 will be wanting from the outermost cycle made up of exosepta. 

 The third complete cycle as understood by Milne- Ed wards and 

 by Verrill is really made up of both tertiary entosepta and of 

 tertiary exosepta. The two kinds of septa are obviously of very 

 different value in their development and relations to the mesen- 

 teries, and, as a matter of fact, will scarcely be of the same thick- 

 ness and radial length to justify their being regarded as mem- 

 bers of one cycle. 



The cyclic formula, as usually understood in systematic works, 

 may be written, 6, 6, 12, X, where A' will represent any number 

 from one to twenty-four. Formulated in this way the number 

 12 conveys the impression that the third cycle is really com- 

 pleted, that all the remaining septa belong to the next or fourth 

 cycle, and that it alone is numerically incomplete. But beyond 

 the two first cycles the septa of the penultimate and last cycles 

 are formed concurrently, or almost so, in pairs, and incomplete 

 cyclic hexamerism, as met with in S. radians, is really an inter- 

 mediate condition in the establishment of two adult hexameral 

 cycles, not of one alone, and attention should be drawn to this 

 in the septal formula. 



According to the relationships above established the morpho- 

 logical septal formula for 5". radians should be written 6, 6, X, 

 6 -|- 6 + X. In this formula the numbers 6, 6, represent the 

 septa in the first and second completed entocycles, and X the 

 number in the third entoseptal or penultimate cycle which does 

 not yet complete the hexameral sequence ; while 6 -f- 6 + X 

 will represent the total number of exosepta, X being the same 

 number as before. In the calice some of the exosepta will be 

 tertiaries and some will be quaternaries, the number of the latter 

 being always double the number of tertiary entosepta. The 

 formula for a corallite having 36 septa would, according to the 

 ordinary cyclic formula, be written, 6, 6, 12, 12, whereas, con- 



