ADULT COLONY. 47 



All the septa of a corallite are well developed, both as regards their 

 thickness and radial extent ; even the members of the outermost cycle extend 

 centrally for about half the radius of the calice, and are but little narrower 

 than the others. The individual septa are usually thicker than the width 

 of the interseptal loculi separating one from another, so that, compared with 

 most other corals, the septal system is very compact, and the calicinal cavity 

 as a whole is correspondingly diminished. The septa vary but slightly in 

 thickness according to the cycle in which they belong. 



The outer septa are not strictly radial throughout their transverse 

 length, but incline along their inner edges and fuse at intervals with the 

 lateral faces of the septa of the second and third cycles, thus forming groups 

 of three, five, or rarely seven (fig. 2, p. 13). The grouping, however, is- not 

 always obvious at the surface of a colony. Thus Verrill (1901, p. 154) states 

 that the septa of the Bahama representatives are nearly all straight and 



FIG. 4. Grouping of the septa within a sextant (i-l) according as one, three, five, or seven septa are present. 

 The Roman numerals l-m indicate the order of the septa and x the exosepta. 



seldom fused. It is, however, always a marked characteristic in sections of 

 the corallum from a little below the surface downward (plate 10, fig. 64). 



When examined in detail the grouping is found to be as follows : The 

 twelve largest septa, constituting the primary and secondary orders, extend as 

 far as the middle of the calice and are united directly with the columella. 

 Of these the six primary septa pass uninterruptedly all the way from the 

 periphery to the columella without any connection with the others except, of 

 course, through union by synapticula. The six secondary septa, on the other 

 hand," are also radial, but with them are united the members of the third 

 order, which never reach the columella. The members of the fourth or oiiter- 

 most cycle fuse one on each side with a tertiary septum. Where only one 

 secondary septum occurs within a primary system the two outermost septa are 

 fused with it, and a grouping of three is produced. Where a tertiary septum 

 and a secondary septum are within a sextant the grouping is in fives, and 

 where two tertiary septa and a secondary occur the grouping is in sevens. 

 The relationships are clearly shown in the series of figures above and also in 

 fig. 2, p. 13. 



