48 SIDERASTREA RADIANS. 



In the course of development of the septa it is shown that from the 

 beginning the exosepta always constitute the outermost cycle, and are fused 

 with the entosepta immediately preceding them. Thus, when only two cycles 

 of septa are present (plate 4, fig. 23), the outer exosepta fuse with the inner 

 entosepta. Where three cycles are developed (plate 5, fig. 28) the exosepta 

 unite with second-cycle entosepta ; where four cycles, they fuse with the third- 

 cycle entosepta. At the early stage of development, represented in plate 5, 

 fig. 28, the second-cycle entosepta are fused with the first-cycle entosepta 

 instead of with the columella as in the adult. The union here takes place 

 in such a manner as to give a decided bilateral symmetry to the corallum. 



The hexameral cyclic plan of the septa is not so manifest in S. radians 

 as in many corals, partly owing to the incompletion of the third and fourth 

 cycles ; but with a little care it can be established by the differences in size 

 of the septa and their other relationships. In mature corallites two inner 

 alternating orders (primary and secondary) of six septa each are always 

 present, while the third and fourth orders are represented by a varjnng num- 

 ber of septa. Owing to this varying incompleteness of the last two orders a 

 study of the septal arrangement yields some interesting results in connection 

 with the manner in which the later additions of septa take place, and also in 

 the relationships of the entosepta and exosepta. 



The hexameral plan is found to be strictly followed so far as concerns 

 the first and second orders, and for the third and fourth orders so far as these 

 are developed. Were the third and fourth orders to be completed the total 

 number would be 48, arranged in the formula 6, 6, 12, 24; but this number 

 appears to be rarely, if ever, reached in S. radians, though frequently 

 exceeded in S. sidcrea. In one of the largest corallites 44 septa were present, 

 but the numbers generally vary from 30 to 36. 



The six members of the primarj^ cycle are readily separable from the 

 others on account of their greater radial extent and thickness ; they pass all 

 the way from the thecal wall to the columella without any of the other septa 

 coalescing with them. They serve to distinguish the six primary systems 

 within which are enclosed varying numbers of smaller septa. The outermost 

 exosepta are also recognizable by their smaller size, one occurring between 

 every two adjacent members of the first, second, and third orders. Less cer- 

 tainty prevails in distinguishing the members of the second and third cycles, 

 though frequently the second-cycle septa are larger and more prominent than 

 those of the third. 



In mature corallites three septa at least are alwaj^s present within each 

 primary system, while there may be five or seven (fig. 2, p. 13). Where only 



