PORITES. 15 
at first sight and to the uninitiated, the possibilities of variation are in reality endless. We 
not only have variations of this simple pattern within itself, that is without altering its 
essential structure, but we have possibilities of slight changes in its essential structure which 
do not seem to take us far enough away to warrant the founding of a new genus. This latter 
restraint is the more felt because it is doubtful how far we can treat the elements of which 
the pattern is composed as morphological units. If the trabecule could be proved to be true 
units, then any change, especially as to number, on the essential plan would have to be 
considered as generic. 
The one difference which has perplexed me most, but which I now think admits of a 
simple explanation, is illustrated by Diagram C, where there is seen, in addition to the typical 
trabecula (w) shown in fig. 2, A, an extra wall trabecula (wr) apparently indicating a 
departure from the fundamental plan. The whole appearance of this new wall, often rising 
higher than that marked w in Diagram A, suggests its belonging strictly to the theca as seen 
from within, and, if so, it seems to indicate a special group of Porites, with three trabecule in 
the dividing wall instead of one. Fortunately, there is a simpler, and, as far as I can see, a 
completely satisfactory explanation. It is reached by reference to an ideal parent calicle 
(fig. 2, D), with the costal edges (c) of the septa sloping down to the edge of the epithecal 
saucer (cp). Each “costa” is only the outer edge of a perforated septum, and is itself per- 
forated, and thus also built up of trabecule with radial and synapticular junctions. It is 
obvious that whether any of these costal trabecule develop or not depends upon the degree of 
crowding of the calicles. When the calicles are quite compact, and the sides are flattened 
against one another, we have the perimeter of the calicle a straight-sided polygon, the wall 
trabecule of adjacent calicles (w in Diagram A) alternating with one another in straight rows, 
the trabecule themselves sometimes even being flattened tangentially. A slightly less crowded 
arrangement is met with when the alternating trabecule of adjacent calicles are arranged in a 
zigzag, as shown in Diagram B. The zigzag is steep or shallow according to the width it has 
to bridge over.* When still greater intervals occur between the calicles, the costal trabecule 
come into play. They appear first in the angles between circular calicles, and then, as the 
width between the sides of adjacent calicles increases, they may appear in straight rows, and 
even in more than one row, producing the condition which we have here called ccenen- 
chymatous. These extra rows of trabecule are so much intervening costal tissue between the 
individuals of a colony. 
One of the distinctions between the Madreporide (excluding Montipora) and the Poritide 
is that, whereas the theew in the Madreporide typically rise as cups above the level of the inter- 
vening tissue, excepting when, as in valleys or round the bases of stocks, they are submerged 
by secondary growth of the latter, in the Poritide no case is known of this occurring. The 
intervening costal tissue is always either flush with the thecze, or, indeed, not seldom rises above 
their apertures. Among the Madreporide, this latter condition is found in Montipora alone. 
* This zigzag wall has been figured by Miss Ogilvie (Phil. Trans., 1896, p. 219, fig. 64 ad), 
but not quite correctly, nor was the true septal formula recognised at that time. 
