40 SIDERASTREA RADIANS. 



granules on the floor of the calice. Probably these differences are determined 

 by the varying conditions of growth of the polyps, whether rapid or slow. 

 The growing marginal corallites are always of the less calcified variety. 

 Such possible variations should be borne in mind, for if extreme coralla only 

 were available for study they might be almost regarded as distinct species. 



Examination of the surface of the corallum, and also of sections, shows 

 that the individual calices of a colony are separated from one another only by 

 fusion of the peripheral edges of the septa. There is no thecal formation 

 distinct from that of the septa. The fusion is complete and continuous, how- 

 ever, and results in the entire lateral separation of one calicinal cavity from 

 another. This is clearly shown in the various figures of the corallum on 

 plate 10 ; and, as further proof, it may be recalled that on decalcification the 

 tissues of the different polyps so far as they are inclosed within the corallum 

 are wholly cut off from one another. 



The septa of adjacent corallites correspond peripherally, end to end, or 

 they may alternate ; and between these two extremes are all intermediate 

 conditions (plate 10, figs. 62, 64). At the periphery the thickness of the septa is 

 usually greater than the width of the interseptal spaces, so that where the 

 septa of adjacent corallites alternate it is clear that a distinct wall of separation 

 is produced between one calice and another ; but where adjacent septa are end 

 to end a shallow interval may remain on each side of the two continuous 

 septa, and the septa are then feebly exsert. 



The exsert condition of the septa is most marked toward the margin of 

 colonies, that is, in the region of new growth. Elsewhere the intervals are 

 generally filled by calcareous deposits, so as to bring the calicinal edge to 

 the same level as the septa. The usual superficial appearance is as if each 

 septum became bifurcated at its outer edge and then each half united with 

 a similar half of two adjacent septa belonging to a contiguous corallite. 

 The result is a very narrow zig-zag partition wall between adjacent corallites. 

 Usually the calicinal boundary as a whole reaches the same height as 

 the septa, but occasionally it is depressed, and the septal edges are inclined 

 towards it. 



HISTOLOGY. 



The different parts of the corallite will be now described, but first the 

 microscopic structure of the skeleton must be briefly considered. In connec- 

 tion therewith the writer has to express his indebtedness to the valuable 

 work of Miss Maria M. Ogilvie (Mrs. Ogilvie-Gordon) who, in her paper, 

 Microscopic and Systematic Study of Madreporarian Types of Corals (1897), 

 has done more than any other investigator to further the histological study 



