114 SIDERASTREA RADIANS. 



septal rudiments, small projections are present here and there on this surface, 

 especially towards the middle of the plate, and some continue growing until 

 they become raised much above the general level of the plate and constitute 

 columellar spines. In the basal plate we have the skeleton in its earliest 

 and simplest condition as a flat deposit, and the septal and columellar forma- 

 tions are vertical upgrowths from it. The latter represent areas at which 

 the calcareous matter is laid down with greater rapidity by the activity of the 

 calicoblasts. The basal plate, however, is covered by polypal tissues on only 

 its upper surface, while the upgrowths from it septa and columella are 

 covered on both sides ; hence the difference in their microscopic structure 

 shown on plate n, fig. 70. There are no axial centers of calcification in 

 the elements of the basal plate, while such are very distinct in the septa. 



When the young corallum is detached from a smooth surface, the lower 

 surface of the basal plate is also smooth and even, but when growing over a 

 rough surface it adapts itself to the irregularities. 



The basal plate increased considerably in diameter from the time of its 

 first formation, keeping pace with the general growth of the corallum. In 

 the early stage on plate 4, fig. 19, it is only 1.3 mm. across, while in the 

 corallum of plate 5, fig. 28, it is 2 mm. in diameter. 



As shown in the next section an epitheca begins to form at the edge of 

 the basal plate whenever growth ceases, while when lateral growth is con- 

 tinued there is little or no possibility of any upturned marginal deposit. 



Apparently the original basal plate of *S. radians never becomes much 

 thickened. Some of the oldest coralla reared rather suggest that the central 

 interseptal portions may be resorbed, or, at any rate, remain extremely deli- 

 cate. When coralla of four months were macerated and separated from their 

 surface of attachment it was found that interseptally the basal deposit had 

 disappeared from the central regions, though retained towards the periphery 

 and along the septa. It may be that on account of its thinness interseptally 

 it had broken away in the process of maceration, while it was supported 

 septally and peripherally. 



In colonial corals the basal plate is generally represented only in the 

 corallum of the primary larval polyp, the later bud polyps not admitting of 

 its formation. But wherever in growing colonies of S. radians the marginal 

 corallites extend beyond the incrusted object a thin parchment-like deposit 

 is found basally. It constitutes the basal skeletal support of the young bud 

 polyps, in the same manner as the basal plate of larval polyps, or as the dis- 

 sepiments at a later stage. 



