56 SIDERASTREA RADIANS. 



in the lamella shown in plate 6, fig. 33, is this basal. The synapticula do 

 not take the place nor assume the function of dissepiments. 



EPITHECA AND BASAL PLATE. 



The most careful observation of the margin of adult coralla fails to 

 reveal the presence of an epitheca or covering of the peripheral septal edges. 

 This is somewhat remarkable considering that such a skeletal formation is 

 developed in yoimg pol3'ps reared from the lan-a, and this whether the 

 polyps are isolated or growing contiguous to others (plates 4, 5). In 

 these larval polyps, however, the extent of the upward growth of the epitheca 

 has been found to vary greatly. In some specimens, apparently stationary 

 as regards growth, it formed a comparatively high external wall to the polyp, 

 wrinkled and diminishing somewhat in diameter from the base upwards, but 

 overtopping all the septa (plate 5, fig. 27), while in others, the largest, 

 actively growing corals, it was either absent or represented only by a low, 

 narrow rim (plate 5, fig. 28). Evidently it is a structure of importance 

 only in the early stages of growth. 



Where, as frequently happens, a part of the growing margin of a colony 

 is free from the incmsted object, a flat, plate-like deposit covers and unites 

 externally the basal edges of the septa. Such a formation by the bud- polyps 

 will correspond with the basal plate of the larval pol^'p, but its peripheral 

 edge is rarely, if ever, upturned so as to cover the free vertical edges of the 

 septa, as in the case of an epitheca proper. 



