20 SIDERASTREA RADIANS. 



termination, where they are all in communication and joined along the poly- 

 gonal boundary lines of the column wall and skeletogenic tissues immediately 

 below. An interval of only about i mm. separates laterally the columns of 

 contiguous polyps. The length of the polyps liberated from the skeleton 

 varies somewhat, but whatever the thickness of the corallum the polypal 

 tissues are always superficial, never extending downward for more than 

 about 5 mm. The largest polyps vary from 4 mm. to 5 mm. in length after 

 decalcification, while young buds extend much less within the corallum. 

 The diameter also varies from 3 mm. to 5 mm., and in mature polyps is prac- 

 tically the same throughout the column. The soft tissues of young buds, 

 before any columella has appeared, are more obconical, that is, longer centrally 

 and shortening as they pass to the periphery. Mature polyps are more 

 cylindrical and terminate somewhat abruptly, all the lamellae being of prac- 

 tically the same vertical length. Owing to the presence of synapticula, the 

 actual base of the individual lamellae is not always as regular or even as in 

 corals resting upon a smooth dissepiment ; the lower surface of a lamella 

 may, in fact, be partly folded round a synapticular bar, as in plate 6, fig. 33. 



The individual interseptal lamellae, like the septa themselves, belong 

 to different cycles and are of different radial lengths. Some are simple, 

 while others are subdivided peripherally into two or three parts. For the 

 greater part of their vertical length, that is, as far as the columella extends, 

 the lamellae are free, either singly or in groups of two or three, but towards 

 the upper extremity they are all joined to one another along their inner 

 margins (plate 6, fig. 34). 



Any simple lamella when seen in surface view appears as a flat, sub- 

 rectangular plate, nearly of the same thickness throughout, and correspond- 

 ing, of course, with an interseptal space (plate 6, fig. 33). It is perforated 

 by rows of circular or oval apertures, arranged roughly in one to four vertical 

 series, the number varying with the cycle to which the lamella belongs. 

 In lamellae extending all the way from the periphery to the columella three 

 or four rows occur, while in narrower lamellae only one or two rows are present. 

 The perforations represent the spaces occupied by the calcareous synapticula 

 which stretch across the interval from one septum to an adjacent septum. 

 Numerous granulations occur on the septal walls (plate 10, fig. 63), but only 

 the large more peripheral ones bridge the interspace between two adjacent 

 septa, and, in doing so, necessarily perforate the soft tissues lining the septal 

 walls and the mesenteries contained within. The smaller granulations merely 

 cause indentations in the lining tissues. 



Transverse sections through the lamellae show that they consist of the 



