MORPHOLOGY OF THE CORAL SKELETON 41 



septum of one calice. The trabeculae in the last-mentioned coral pass 

 upward and slightly inward from a vertical line that indicates the limits 

 between two adjoining calices. The line of divergence in highly arched 

 septa, such as is the case in DicJioccenia stokesi M.-Edw. and H.,^ often coin- 

 cides with the vertical axis of the arch. 



Thus, it seems that the relations of the septal zones, one on each side, 

 to the line of divergence depends on certain mechanical conditions In a 

 septum with a nonexsert margin and a small costal development, one would 

 expect the outer zone to be narrow or even suppressed altogether. 



SIMPLIO AND COMPOUND TRABECULAE. 



A simple trabecula may be defined as a trabecula composed of a single 

 series of simple calcification centers. Compound trabeculae may be of two 

 kinds. The Mussa septum may be taken as an illustration of the first." A 

 trabecula does not arise at a line of divergence running parallel with the 

 longitudinal axis of the septum. The septal margin presents large spini- 

 form teeth. Each one of these teeth nossesses its line of divergence, so 

 that each tooth has a structure analogous to that of such a septum as 

 Dichocoenia (Galaxea is the coral with which Ogilvie draws comparison).^ 

 These compound trabeculee have their origin at a line of divergence corre- 

 sponding in position with the wall. The second type of the compound 

 trabecula may be illustrated by Orbicella (Heliastraea, auct.). Ogilvie has 

 described the struct^ire of the Orbicellan septum in her work already 

 quoted (pp. 139-146). It differs from Mussa in having smaller compound 

 trabeculse, and a calcification center is sometimes composed of groups of 

 fascicles which radiate outward from difterent points in the center. The 

 calcification center itself is compound. To enter into an elaborate discus- 

 sion of these features is somewhat beyond the purpose of this resume'. 



DIRECTION OF THE TRABECULyE. 



The inclination of the trabecular with reference to the longitudinal axis 

 of the septum is extremely variable. In many corals the angle between 

 them is at first small, but as growth proceeds the inclination becomes 



greater. (Compare PI. I, figs. 2 and 4.) 



_^ « 



' See p. 140 of this paper. 



2 Ogilvie, Microsc. and Syst. Stud. Mad. Types of Corals, pp. 128-139. Cf. fig. 14, p. 123. 

 ^ Ogilvie uses the word homologne where I have used analogue. Her use of the word homologue 

 seems to me loose. 



