35 
this portion of the corallum are also regularly polygonal (triangular, 
hexagonal, heptagonal, and pentagonal), and variable in size, and are certainly, 
as a rule, in close contact. On the other hand, in sections tangential to the 
branch, and taken a little below the surface, the tubes are cut across on their 
outer portions, where they are periodically thickened. The tubes still appear 
to he polygonal and in contact, each being bounded externally by a well-marked 
dark line ; hut the appearances presented by the area within this houndary- 
line apparently vary according as the section traverses the tubes at the level of 
their thickened portions, or at that of the unthickened intervals between the 
latter. In the former case the visceral chamber is seen to be greatly con- 
tracted, and may even be reduced to a comparatively small rounded or sub- 
polygonal central tube, which is, in turn, surrounded by a thickened ring of 
sclerenchyma, which usually shows distinct traces of its being composed of 
successively-deposited concentric laminae. In the latter case there is still a 
ring of sclerenchyma within the dark outer polygonal boundary ; but this 
ring is of small thickness comparatively, and the central tube is wide and 
open. The walls are thickened at short intervals by annular accretions of 
growth, the portions of the tubes between them retaining their normal 
diameter. These thickened portions are usually placed at corresponding 
levels in all the corallites. 
In many parts of tangential sections the corallites exliibit few features 
that would satisfactorily separate them from similar sections of certain 
Monticuliporidm, though they usually have exceptionally thick walls, and 
often exhibit a dark ring a little within the true wall, and concentric with 
the latter. 
Longitudinal sections of the corallites show the periodical annular 
thickenings of the tubes in a very instructive manner, and demonstrate that 
these are really thickenings of the wall, projecting both externally and 
internally — in fact, the longitudinal section of the wall has a regularly 
moniliform appearance, owing to its successively traversing thickened and un- 
thickened segments. Sections of this kind also show that there exist remote 
O 
and usually complete tabulm, which are generally placed at approximately 
corresponding levels in all the corallites of a single colony. 
Such being a general view of the corallum of Stenopora, we may now 
consider some points of its structure in detail. The secondary deposit which 
forms so conspicuous a feature in the tubes of most New South Wales and 
