148 PROF. p. M. DUNCAJSr ON THE 



local and general radiation from the median line of the septum 

 or from one edge. Moreover, these fibres are of different lengths. 

 Hence where the fibres are not mostly fusiform, but have rounded 

 ends or geometrically shaped terminations, they will act diff'er- 

 ently on light during its transmission through parallel planes and 

 radiating series of them. 



The common appearance is of excessively minute dark cross 

 markings, which, under a high power and careful illumination, 

 resolve themselves into more or less circular rings placed nearly 

 in little linear series. The rings are dark, and have a light 

 central part. And it appears to me that these markings are caused 

 by the shape of the larger ends of the fibres below or above those 

 iu focus. 



The thinner septa, which are so constantly seen perforated and 

 ending above in a wave-like edge, explain the construction of the 

 hard parts better than the sections of the larger septa. Thin 

 septa are so delicate at the free edge, that they may be examined 

 successfully when mounted iu balsam without rubbing down 

 previously. 



The direction of the fibrous structure is exceedingly irregular ; 

 and it does not appear that there are definite vertical sets of 

 fusiform bodies with off'shoots here and there forming the sides 

 surrounding the vacuities. There is nothing resembling microsco- 

 pically the lattice-work structure seen with a low power in 

 Porites, for instance, amongst the Perforate corals. But near 

 the free edge of the thin septa the fusiform bodies and long 

 tapering prisms project with a sharp end outwards, and are placed 

 side by side and in series of planes one over the other. A little 

 lower down these microscopic elements become oblique, and 

 those on one side of a line which corresponds with the apex of a 

 dentation on the free edge of the septum converge towards those 

 on the other side, a Vandyke or herring-bone appearance being 

 given. Polarized light is a great assistance in this research. 

 ■ In the thin portions of the youngest septa where there are 

 fenestrations, the fibrous element does not radiate from or to 

 them from denser parts of the septum. In many places the fibres 

 surround, and have their long axes parallel with, the curves of 

 the periphery of the openings ; but here and there a dark line or 

 lines which pursue an irregular, yet on the whole radiating, 

 course from an opening have the fibres converging to them ob- 

 liquely. On employing as high a maguifying-power as the section 



