292 Dr. H. A. Nicholson’s Contributions 
developed in variable amount, and often appear in clusters. 
Tangential sections taken a little below the surface show, fur- 
ther, that the corallites gradually become lessand less thickened, 
and more and more polygonal in outline, while traces of the 
original lines of demarcation between adjacent tubes can be 
made out, and the spiniform tubules gradually disappear. In 
the axial region of the corallum, as shown in longitudinal or 
transverse sections (fig. 3, B), the corallites, finally, become 
completely separate, having now thin walls and a polygonal 
outline. 
i B 
Monticulipora? tumida, Phill., from the Carboniferous rocks of 
Redesdale, Northumberland. A. Part of a tangential section, enlarged 
twenty-four times, showing the large and small corallites, and the rows 
of interstitial spiniform tubules. B.- Part of a transverse section, enlarged 
twenty-four times, showing the polygonal and thin-walled axial tubes 
and the thickened walls and complete tabulz of the peripheral portions 
of the corallites. 
The only two remaining structural points to notice concern 
the nature of the tabule and the thickening of the walls of the 
mature corallites. As to the first of these two points, it is 
quite certain that the tabule are essentially and habitually 
complete and imperforate (fig. 3, B). In no single longitudinal — 
or transverse section have I ever detected an incomplete 
tabula, and out of a large number of tangential sections I have — 
only once met with an instance of a perforated tabula, and 
then only in one or two corallites. 
As to the second point, the walls of the corallites in the 
peripheral region of the corallum are invariably greatly thick- 
