PORITES, 13 
hitherto there has been no possible means of deciding to which genus they belong. A 
comparison between the two septal formule will now enable us to decide. If the twelve 
septa conform to Diagram B, then it is a Porites; if, however, it is an irregular reduction of 
Diagram A, it is a Goniopora. The irregularity is seen. most frequently in apparent forkings 
ot the septa before they reach the wall—such a forking as, for instance, we should get if, say, 
in Diagram A any of the tridents made by the 2nd and 3rd cycles lost their middle prongs. 
The typical septal formula of Porites is that shown in Diagram B, that is, if we omit the 
rudimentary tertiaries inserted in the drawing for the purpose of making the suppression clear, 
If the reticular theca is regularly built, we can trace two systems of elements in its 
structure ; firstly, the vertical bars, to which the name trabecule is here strictly confined ;* 
and secondly, the horizontal bars, or horizontal elements or junctions, as they are called in 
this book. These latter are again divisible into («) radial bars, original components of the 
septal plates; and (0) concentric bars, usually called “synapticule.” 
Fic. 1.—Diagrams to show how the septal formula of Porites (B) { may be derived from 
that of Goniopora (A) by the degeneration of the tertiary septa. 
The whole skeleton is built of these elements; their various groupings, comparative 
developments, and appearances at the growing surface supply us with all the systematic 
‘characters which the skeleton affords us. Whether these elements are true morphological 
units, or mere arrangements of tissue, is a difficult question, which I think must be decided 
in favour of the latter alternative.t But the point may be raised as to whether even what 
are mere arrangements of tissue, if repeated often enough, do not- gradually acquire a 
morphological significance of their own, and whether, in the present case, this has not 
happened. This is a point which the next comprehensive study of the genus must decide. 
The attempt to do so here is out of the question. This volume is now complete; it has 
* On the earlier use of this term, see Vol. IV. pp. 4, 18. 
+ See remark on this Diagram on p. 24. 
} This point was discussed in the Journ. Linn. Soc., xxvii. p. 138. 
