Il6 SIDERASTREA RADIANS. 



elevated beyond the level of the septa (plate 5). Where two or more polyps 

 developed contiguous to each other a common epitheca was formed along the 

 line of contact (plate 5, fig. 30), but each became distinct later. In one 

 instance two polj'ps, along with their septa, were embraced in a common 

 epitheca. 



This surprising variation in the amount of epithecal deposit appears to 

 be determined by the rate of growth of the polyp. As already noticed, the 

 polyps varied greatly in this respect, and the epitheca was best developed in 

 individuals which increased but little in size. The transverse narrowing in 

 the later stages may be taken to indicate that the polyps had become some- 

 what less in their basal diameter. Where a polyp was growing rapidly, 

 enlarging the diameter of its basal disc and extending the septa peripherally, 

 it is manifest that an epitheca could not be formed, or, if formed, would need 

 to be resorbed. In the two coralla represented on plate 5, figs. 28, 29, there 

 is practically no epitheca, but the basal plate is much thicker in its peripheral 

 half than in the middle. Presumably, the polyp did not rest long enough at 

 any one stage to permit of the secretion of an epitheca. Until maturity, 

 therefore, it is possible that individual corallites of the same species of 

 coral may be provided with or be destitute of an epitheca, according to the 

 slow or rapid rate of growth of the polyp ; the morphological value of the 

 structure becomes somewhat lessened when its formation is shown to be 

 dependent to such an extent upon physiological conditions. 



The peripheral skeletal formation in Siderastrea is of interest in con- 

 nection with the much discussed question as to the nature of the thecal and 

 epithecal wall of corals. In some respects the structure recalls that described 

 as theca (Mauer) by von Koch (1897) in his paper on the development of the 

 skeleton of CaryophyUia cyathus, but a comparison at once establishes their 

 different values. In both species the structure in question arises as an inde- 

 pendent peripheral part of the corallum, and narrows from below upwards. 

 But in the species investigated by von Koch the annulus is inclosed on both 

 its inner and outer sides by an upgrowth of the basal wall, while in Sider- 

 astrea it is external from the beginning, only covered by the polypal tissues 

 on its inner wall and growing margin (plate 9, fig. 53). Again, in the first 

 the deposit early unites the edges of the septa, but in the other it remains 

 entirely free from septal connection for a long time, and then only joins their 

 edges as an independent external covering. The different character of the 

 theca (pseudotheca and true theca) in various Madreporaria has already been 

 alluded to (p. 45), and the original peripheral structure arising in CaryophyUia 

 would certainly belong to the "true theca" type. Miss Ogilvie (1897, p. 



