210 MARIA M. OGILVIE GORDON. 
From the excerpt, it might be concluded that I supposed 
the middle region of the septum was always occupied by 
black organic matter. On the contrary, I examined my 
slides both by transmitted and reflected light, and knew very 
well that in some cases the middle region of the septum 
appeared lighter. But, as I wished to point out, there were 
cases where bright coaly specks were undoubtedly present, 
and would justify the term “dark points” met with so often 
in the literature of Madreporaria. Yow 
In my work I accepted Dr. Bourne’s term ‘centres’ of 
calcification” for the “dark points” expressly because it 
assumed nothing with regard to the actual condition of any 
deposit that might originally, or by secondary changes, be 
present at the “centres” of calcification. My aim, in the 
passage to which Mr. Duerden made reference (aut., p. 127), 
was to demonstrate the frequent presence of organic matter 
at the septal axes or centres of calcification, and to identify 
it with the organic residue in the skeletal units or 
calcifying calicoblasts which composed the lime-form- 
ing skeletal layer, doubled at the septum. 
I wrote, “The appearance of the ‘dark line’ with trans- 
mitted light, although generally opaque, is not always so... . 
The ‘ points’ in a transverse section of Galaxea, for example, 
appear at one place homogeneous, and yellowish or dingy- 
brown in colour; in another place the ‘point’ seems a fairly 
large, circular area, filled with granular, powdery material, 
and then it is usually dark.” Again, “In all cases we have 
simply to do with centres and axes (ideal) of calcification, 
around which the calicoblasts are grouped in the living polyp, 
and from which therefore similarly oriented fibres ultimately 
radiate when complete calcification has taken place.’ In 
explaining lateral ornamentation of the septa, I wrote, “ Small 
pits are present on the ectodermal, skeletal-producing surface. 
Subsequently the skeletal layer of the septum is an exact 
cast of the form of the ectodermal flap.... The com- 
ponent calcified calicoblasts of the layer have their 
fibres set at right angles to the sides of the pit, and the 
