MADREPORARIA. 
V. THE AFFINITIES OF THE GENUS. 
These were discussed and arranged in tabular form in Vol. IV. p. 29. 
here been amplified and amended, 
The scheme has 
A primitive porous Coral, that is, one with a parent form in which the epithecal cup, or the proto- 
theca, is flattened out, and the secondary theca is built of septa joined by synapticule. 
With many cycles of septa 
and usually large calicles ; 
with rapid growth in height 
of the secondary perforated 
theca, resulting in simple, or 
simply branching forms. 
THE EUPSAMMIID A. 
Usually with two cycles of 
septa. with small calicles, and 
with rapid growth in height 
of the secondary theca, 
resulting frequently in luxu- 
riantly branching forms. 
THE MADREPORIDA. 
Septa very perforate, in two 
to three cycles; the secondary 
theca shows no rapid growth 
in height, but remains basal 
and disc-like ; hence the 
colonies are astreiform. 
THE PORITIDA. 
With three cycles of septa, 
GONIOPORA. 
| 
By suppression of one cycle, 
PoRITES. 
In the final column I have left Goniopora arranged as if it were the ancestral form to 
Porites. I do not mean, thereby, that it necessarily was so. From a purely ideal morpho- 
logical point of view, it might have been, but, so far as actual knowledge of facts can carry 
us, we might just as well have arranged Goniopora and Porites side by side, the former with 
three and the latter with two cycles of septa, both as derivatives of some primitive Poritid. 
I have shown, in fig. 1, Diagram B, how the tertiary septal formula of Goniopora might have 
been reduced so as to give rise to the formula of Porites. But we have already emphasised the 
fact that whenever the septal formula of Goniopora suffers reduction in life, it is always very 
irregularly. In Porites the formula is so regular that we can hardly believe that it was due to 
any such irregular method of reduction; we have rather to believe that Porites never 
possessed the third cycle at all, and therefore, if it is derived from Gonzopora, it is by the 
fixation by early maturity of a young stage, when only two cycles were developed. 
VI. THE DIAGNOSIS OF THE GENUS. 
Porites consists of minute Poritide, with twelve septal plates rising above a flattened 
epitheca; two of these are directives, which, with a columellar tubercle, divide the calicle 
bi-symmetrically. These radial plates are joined by a varying number of synapticular bars, and 
are themselves typically extensively perforated, and acquire all the appearance of being lattice- 
work, sometimes so regular as to appear as if built up of vertical and horizontal bars. The tops 
