Il8 SIDERASTREA RADIANS. 



extending as far as the center of the calice (plate 5). Some of these upgrowths 

 appeared as separate projections from the basal plate, as if produced by special 

 invaginations of the basal wall of the polyp the refoulement columellaire 

 of Delage & Herouard (1901, p. 558) ; others seemed to be direct continua- 

 tions of the septa, not requiring a separate upfolding of the basal disc. The 

 former undoubtedly represent a true independent columella, while the latter 

 by their union might give rise to a so-called pseudocolumella. At no time, 

 however, could any sharp distinction be drawn between the septal and the 

 independent upgrowths, except as regards their position. In the coralla rep- 

 resented on plate 4, figs. 23 and 24, two or three of the middle granules are 

 obviously distinct basal formations, while exactly similar spinous projections 

 occur at the end of some of the septa. The middle of the corallum on plate 5, 

 fig. 28, is also occupied by distinct spinous upgrowths, both basal and septal in 

 character. 



A further stage in the columellar growth is represented in the coralla of 

 figs. 28 and 29. Here the intervals between the spines are becoming partly 

 occupied by the deposition of secondary calcareous matter, so that their act- 

 ual origin is obscured. In the mature corallum it was frequently found 

 (p- 53) that the columella is minutely spinous or papillose as seen from the 

 surface, and that in sections for some distance below it remains spongy, 

 becoming compact in the deeper regions by the later deposition of calcareous 

 matter. Further, where the coralla are very strongly calcified the spinous 

 character of the columella disappears even superficially, the interspaces 

 being altogether occupied by the secondary deposit, which keeps pace with 

 the septal and columellar spinous formations. These differences in the 

 mature calice thus coincide closely with those represented in the larval cor- 

 alla of figs. 23 and 29. 



Both from its origin and mature characters the columella of S. radians 

 is therefore formed from three independent sources: (i) Separate basal 

 upgrowths; (2) septal spines, continuous with the central ends of the septa ; 

 (3) a secondary deposit filling up the interstices between i and 2. Histolog- 

 ically the trabeculae of all three are alike (plate 10, fig. 65). 



The columella of Siderastrea is thus a " true " columella, to be distin- 

 guished from a " false " or " pseudocolumella " where there is no direct basal 

 upgrowth, but the entire structure is formed from the inner septal edges. In 

 its development it agrees most closely with the account which von Koch 

 (1897, P* 769) gives of the formation of the columella in Caryophyllia. 



