ADULT COLONY. 45 



center of calcification, and from it fascicles of fibers radiate on both sides, the 

 black, finely granular matter extending but a very short distance. Bach 

 center of calcification, therefore, represents a single growth period in the 

 upward progress of the septum. They are distant from one another about 

 0.013 mm. A trabecula is thus a vertical series of fibrous groups, each group 

 of fibers being deposited around a distinct axis of calcification. A trabecular 

 part is the name given to the separate groups of fibers formed by the fasci- 

 cles at one center of calcification. Bach stria seen under low power on the 

 surface of a septum represents a single trabecula, and the tooth in which an 

 opposite pair of striae terminates at the edge of the septum is the growing 

 apex or organic center of the trabecula. Around this organic center new 

 calcareous matter is constantly being added by the activity of the calicoblasts. 



WALL OR THECA. 



The wall or theca of a simple coral or of a corallite is defined by Vaughan 

 (1900, p. 48) as "that part of the skeleton that cuts off more or less com- 

 pletely the interseptal loculus from peripheral communication with the 

 outside." In Siderastrea the thecal wall at first appears indistinct, as if 

 represented only by the united peripheral edges of the septa of adjacent 

 corallites. Some of the septa of contiguous calices are in the same straight 

 line, in which case it is difficult to distinguish just where the calicinal 

 boundary comes ; in other places a septum of one corallite corresponds with 

 an interseptal loculus of the other, when the distinction between the two 

 calices is quite clear. Usually it appears as if the septa were forked 

 peripherally, and the two limbs of one septum unite in a zigzag manner 

 with two adjacent septa in the next calice. 



The nature of the wall is best seen when a corallum is fractured verti- 

 cally, and adjacent corallites can be viewed lengthways. As indicated on 

 plate 10, fig. 63, the theca is then represented by a narrow, vertical, continuous 

 ridge, which completely separates the interseptal loculus of one calice from 

 that of the other. Further, in the section shown on plate n, fig. 67, this 

 vertical ridge is found to consist of a distinct trabecula, and from it the 

 trabeculae of adjacent septa diverge. 



Miss Ogilvie (1897, p. 180) has given a figure of Siderastrea (sp. ?) 

 somewhat similar to that of plate 10, fig. 63, but the thecal wall (pseudotheca) 

 is there represented as a discontinuous ridge, as if comparable with a row of 

 synapticula, and in such a way as would permit of communication between 

 one calice and another. This, however, is not its condition in S. radians. 

 In all instances I have found the wall to be continuous, and the fact that 



