'SG BE. G. C. BOURNE ON 



visible on the under surfaces of the adult specimeus of 

 T. hastatus. 



Associated with the filling up of the scar is a considerable 

 extension of the epithecal deposit on the lower surface, 

 obliterating the costse, and giving the same nacreous lustre 

 to this region as is seen in the adult specimens. This specimen 

 was apparently dead when collected and had suffered some 

 erosion and injury, for the interseptal loculi are partly filled 

 up with Poraminifera, and the upper parts of several of the 

 septa are broken off. I am able, however, to count thirty-six 

 septa, whose arrangement is identical with that of the first and 

 second specimens, but they have progressed much further in 

 growth. 



The uninjured septa, as is shown in Plate 6. fig. 11, project 

 well above the margin of the calycle and are of unequal sizes. 

 Those of the first order are the most exsert, then follow those of 

 the third, then those of the second order, and the quaternaries 

 are the least exsert of all. The inner ends of the septa are 

 so much damaged that it is difficult to say anything certain 

 about the presence of pali, but there are indications of a 

 prominent inner spine separated by a notch from the main part 

 of the septa of the first and third cycles, so I am inclined to 

 think that the pali are being developed at this stage. The 

 specimen, however, is of great interest, for it clearly indicates 

 that the greater part of the corallum of the adult Trocliocyathus 

 hastatus, with its characteristic arrangement of septa and pali, is 

 formed by the upgrowth of the septa from the scar of detachment 

 on the upper surface. In the order of their appearance the 

 septa follow the law of Milne-Edwards and Haime ; that is 

 to say, that in each of the three first orders the septa appear 

 simultaneously and form three complete cycles. The septa of the 

 fourth order are twelve in number and appear in the interseptal 

 loculi 1 + 3, 3 + 1, in each system forming an incomplete cycle. 

 The septa of the fifth order appear very late in the interseptal 

 loculi 3 + 2, 2 + 3, in each system, and with the septa of the 

 fourth order form a complete fourth cycle. But during the 

 o-row th from the young to the adult form, the rate of growth 

 of the septa does not follow this law. The six primary septa are 

 always predominant in size, but the secondaries, which in adult 

 specimens are equal or nearly equal to them, lag behind in the 

 earlier stages of regrowth, and are surpassed by the tertiaries. 



