46 SIDERASTREA RADIANS. 



when a colony is decalcified adj acent polyps are found altogether cut off from 

 one another would prove that their cavities are not in lateral communication, 

 as would be the case were the boundary wall interrupted. 



The terms psc lido/ krca and eiitheca of von Heider (1886) are well known 

 to coral students, the former referring to a theca produced by the septa 

 becoming so much thickened peripherally that they fuse together, while the 

 latter is applied to a theca formed from separate trabeculse between the 

 peripheral ends of the septa. Vaughan (1900, pp. 48-52) has considered 

 very fully the value to be applied to these distinctions, and comes to the 

 conclusion that " From the great variation not only in the same species, but 

 in a section of a single corallite, no special systematic importance can be 

 attached to the theca being of the so-called true or false variety. True theca 

 marks the Anlagen of new septa, as von Koch and Bourne have shown, or 

 occurs in calices where the septa are distant from one another and their outer 

 ends are not sufiiciently thickened to effect peripheral fusion." 



The theca of Siderastrea is described by Miss Ogihae as a pseudotheca, 

 and, as above shown, it is certainly formed at the peripheral extremities of the 

 septa. On the other hand one, or rarely more, distinct centers of calcification 

 occur laterall}' in the interval between one septum and another, though not 

 to be distinguished from those of the septa except by being inclined to them at 

 an angle. In her figs. 43(2, 43*5 (p. 181), Miss Ogilvie represents such centers 

 of calcification, but regards them as synapticular. Although presenting the 

 same appearance in transverse sections, plate 10, fig. 63, and plate 11, fig. 67, 

 show that this is not their true character ; rather, the thecal trabeculse are 

 continuous vertical partitions. According to the accepted definitions it would 

 seem that the theca of Siderastrea should be regarded as a true theca, having 

 a separate trabecula between the ends of adjacent septa from which the tra- 

 beculas of the septa radiate. 



An independent thecal wall, however, is found to be wanting in the 

 corallum of larval polyps, the epitheca being in no way comparable with this. 

 In the simple corallum, as far as reared, the septa remain free and exposed 

 peripherally, except in so far as they may be imited by synapticular growths 

 or covered by the epitheca (see plates 4 and 5). Likewise the outer septal 

 edges of the marginal polyps in a colony extend for a considerable vertical 

 height, exposed all the waj^. 



SEPTA. 



The detailed characteristics of the septa have been already described, so 

 that it only remains to discuss their arrangement and relationships. 



